1. Use the REVOKE command as detailed in the mysql docs
http://www.mysql.com/doc/en/GRANT.html An advantage of this is that the changes are noticed immediately, however the user is still in the table (but if you 'REVOKE ALL PRIVILEGES', then they have no rights).
2. Go to the tables (and if you've granted btoh global and database privileges, there will be more than one table to look at) directly and execute DELETE statements to completely remove the user. You'll need to FLUSH PRIVILEGES when you are done with this method, but on the other hand, ALL traces of the user are removed.
I've never actually done this. Would there be any repercussions for rollbacks/recovery (ie: if a user did something and you then removed him completely, could you rollback/recover something he did?)