Easiest way for me to do it is just to remove all information associated with it and leave it a generic stub.
The information you control is password and email and signature text. The information you don't control is the username itself. I normally replace that with "user12345".
The result of that is something similar to a reddit "deleted account" entry the one occasionally stumbles across.
--
Oh, what the heck. 1PLXzL1CBUD1kdEWqMrwNUfGrGiirV1WpH <= tip a hedgewars dev