A good set of general guidelines can be extended from the USENET guidelines. It's generally a good idea to listen to what is going on in a group (or sometimes what isn't going on in a group) before you start sending out tons of messages. It's generally not acceptable to waste message space (typing a sentence one letter per-line, pasting in file dumps, things like that), but it can be ok in some groups, so you should always ask first. It's also generally not a good idea to harass anyone on the net, and thus on ICB.
Since ICB is a real-time communication environment, you don't really have to wait days or weeks before you start to talk in a group. But you should try to get some idea of what the current topic of conversation is before you decide to steer it to your own conversation (one notable exception is that if you need help with something, it's usually ok to ask). Look at it in a similar way that you would personal conversation: if you were in a group of your friends and talking about something, and an outsider walked in to the middle of your group and started spouting off about something completely different, that sort of interruption would be considered incredibly rude. The same is generally true on ICB. It's even true if everyone is quiet. In this case, look at it like a study group. You're all quiet and sometimes conferring with each other, and a loud outburst by an intruder would also be considered rude. On the other hand, if your intent is to be rude, you don't really have a right to complain if you get booted.
If you were trying to follow a story or conversation in a group, and someone started to fill up your screen with lots of garbage, it would make following your conversation rather difficult. If what they're doing follows another thread of a conversation, then that's usually acceptable. But doing this for no reason, or in a group of people that haven't expressed interrest in your latest text-graphics-images, would also be rather rude. This is the reason that it's generally rude to waste lots of screen/message space with file dumps and the like. It makes other people's ability to follow the conversation much more difficult. In general, it's best to ask the group if they mind you dumping in the file or text-graphics, or command output. Keep in mind that the current clients support "personal-to" mode, where all of your un-/m-ed messages get /m'ed to a particular user. So, if you just want to show that one user the file/image/output, you don't need to dump it to the group.
Harassing people on the net is just as bad as harassing them in person. I'm not sure anything more than that needs to be said. However, don't think that because you're on a computer that the other party has no recourse. Computer harassment is a prosecutable offense.
In general, you start out on ICB in a public, unmoderated group. If you don't like what is happening in such a group, you can leave and make your own group, but you can't stop the course of a group you aren't moderating. When you create your own group, if you choose to moderate it, you basically have decided you want to avoid something that was happening in other groups. As a result, it's only logical that you can establish the appropriate set of behaviors for the group you are moderating. In order to make sure this happens, you have 4 things you can do. You can boot people who break your rules, you can keep people out by making the group secret, or invisible, or restricted. Just because you choose to only do a subset of these doesn't mean that you're less entitled to your rules. Some people seem to think that they have more, or as much, control of a group that the moderator. This is only true if the moderator says so.
So, if you break some general guideline (like the ones I said above), or something more specific that the moderator created their group for, then don't be surprised if you get booted from the group. It's not the moderators job to entertain you, or accommodate you. It's your job to either fit in to existing groups, or exercise your privilege to make your own. That may seem a little authoritarian, but remember that if any group is too authoritarian for you, you can create a less authoritarian one. If other people agree with you, they'll follow. But, if instead we gave moderators no power over their groups, then what recourse would an uncomfortable user have? If they left, they could be followed by the very people that are annoying them. (Hopefully, you'll get a warning before you get booted. And if you suddenly see east coast people saying "p" and or "q", they're voting "thumbs down" to get whoever is currently being annoying booted. If you think that might be you, it might be a good time to start apologizing)
I realize that your reply might be "Then they could log off of ICB". However, I don't believe that a calm user should have to give up their privilege to converse with their friends because of annoying users. I think they have the right to both use ICB and take steps to keep annoying users from bothering them (short of banning those other users from ICB) (note, banning a user from ICB is not the same as booting them from a group on ICB). If you think that is censorship, then all I can say is that booting someone from a group doesn't keep them from talking to anyone, not even the other people in the group. It just keeps them from doing it IN that group. You have the right to speak, but others don't have the obligation to provide you with a place to speak. The same is true on ICB. You have a right to send messages to people, but people don't have to let you in their groups to do so (nor does your right to send those messages require being let in to their group).
On a slightly lighter note, many ICB people use ICB to avoid what they perceive to be the "IRC" culture. Actually, many of them are avoiding the inrush of new-to-the-net people that come with an ever larger Internet, and some of those people didn't like the idea of this home page being written. However, because so many ICB users don't like IRC, it's generally not a good idea to ask how to get to IRC, why we don't use IRC, or why this doesn't work like IRC. The answers are typically "If I knew, I'd try to forget", "We don't want to", and "We don't want it to", respectively. To many ICB users, IRC is a four letter word. Try to keep that in mind. :-)
It is also VERY much against the rules to fake your userid when logging in to ICB. It's not cute, it's not a complex thing you can show off (it's about as impressive as being able to blow your nose). Some server admins have said in the past that they'll lock out sites that allow it.