This has been [proposed many times](http://meta.stackoverflow.com/questions/61468/is-it-possible-to-flag-a-user-rather-than-just-individual-posts), and [denied][1]. 

If you want to bring an abusive user to the attention of a moderator, flag *one of their posts* for moderator attention and describe what's going on in the field provided. Moderators are perfectly able to follow the link from the post to the user, and can then quickly deal with the account.


  [1]: http://meta.stackoverflow.com/questions/1323/flag-abusive-users