2 FAT bugs detected

When I delete a user that has already signed up for things:
    + User's PK information is left behind in the events he/she has signed up
    for.
    + This raises errors when we try to query for each PK in that field.

I think that django is re-using old deleted PK's.
I need to verify that this is truly a problem.
Possible Solutions
    + Add a cleanup function that gets called when a user is deleted. This
    would remove all references to him/her in the database, which would "clean"
    that PK for future users
    + Find a way for django to keep incrementing user PKs even if there are
    lower unused numbers. This could be an issue that comes from sharing auth
    between LDAP and local.
    + Make it impossible to actually delete User objects, and make it so you
    can only disable them.