Well, heres an idea. What about a report button that would tell moderators about an abusive user? Most sites have it, so you know it is effective. For example, if you clicked the “report” button, there would be a message in a special “moderator only” chat room. . The mod could then look at the recent (not the full) chat log of that player and decide what action shall be taken.
I have made a (rubbish) draft of what it would look like, here:
EDIT: Yet another update to it!
Please take this idea into consideration!