no you cant ban someone (just THINK of how that feature would be abused!!) you can however block them from sending you messages. you can also remove them from your fanlist which should stop them from being able to contact you via IM. you can report them and put an explanation in the notes section as to how this user is "abusing" you.
here are a few links that should help
+what happens when you report
link
+why you should use the notes section
link
+how to drop a user from your list, the profiles of the f4 and how to block someone
link
if none of this works dont attempt to take matters into your own hands. ask the f4 for help! be precise in explaining what this user is doing dont just write them a message saying: OMG DYS USER IS LYK SOOO MEAN because chances are that will be ignored. instead, write a PM to either papa or dave in proper english (not textspeak)giving a detailed explanation of what happened. if this is a personal fight aboot some shipping war they might not be able to help but if this user is violating the terms of service the f4 could either warn then or ban them.