The MoinMoin instructions for how to delete a user, while correct, suck. First find your wiki settings file (mine is in /etc/moin/mywiki.py). In that file find your 'data_dir', mine was pointing to '/var/local/somewiki_wiki'. The user files sit in '/var/local/somewiki_wiki/user', one for each user. Grep that directory for the name you are looking for.
Delete the appropriate user file and also delete the cached username mapping file in ../cache/mywiki/user/name2id (this will be regenerated by django). Restart apache.