I know this is an old issue but...
You should be able to manually purge the data from your files.
Not entirely sure where it puts new users data but find that, remove the relevant bits and it should work.
Make a backup first just in case.

Yeah, it seems to just remove them from the users table. Looks like the owned_adoptables, messages and other tables stay the same I guess. Might be something HoF needs to change for the next version. Until then, manually removing them from all the tables in the database works

Far as I know, its a similar problem for all adoptables sites in any script. They need databases and there doesn't seem to be an automatic way of purging a user. Maybe it isn't possible. I would have thought it would already be coded in if it was. Maybe someone hasn't bothered fiddling the codes to do it though.

It's possible, it's just deleting all rows in all tables that use users, by the user ID/username. A database is just like an advanced text file that stores information, you are free to edit, add and delete whatever ^^ You just need to worry about data integrity

Some programmers/site owners choose not to delete rows from certain tables to preserve information/records of the user and just put them "inactive".

I think user references are kept by username in other tables? If so... We can either just delete every record of the user. Or if keeping messages and adoptables is important for statistics, instead of username use ID to attach a user to rows, since ID will always be unique, even if a user is deleted no one else will use that ID

Of course, in the new version, owned adoptables by deleted accounts will be either purged or moved to pound center. However, you can only do this if you delete user accounts through ACP. If you use PHPMyadmin, you have to clean up tables for owned adoptables yourself.