Typically: Other chat sites, or other advertising which serves one's own interests (i.e. one's own youtube channel), or advertising of questionable content. Especially if you're just logging into rooms to paste a link and leave. It's best to check in with a moderator or administrator, or post on the forum to ask if it's ok, before proceeding with any actual advertising.
Ren If you've read the rules, you'd see written at the bottom that you can have your friend send an e-mail to our support team. It is likely a rule was broken, considering bans do not happen for no reason. Thanks!