[Koha] Patron Deletions

We use Koha in a Prison Library in the Education Unit managed by Tafe staff, and have been unable to delete patrons from the patron file once these people have left. We are currently using Version 2.2 We select the patron record, select delete and select OK to the message Are you sure you want to delete this patron. This unfortunately does not remove the patron - they are still listed, even though the patron has nothing outstanding. Has anyone else experienced this problem, or are we missing a basic step??? Kay Smith Regional Services Librarian Bendigo TAFE