Can't use email in a new blog after deleting old blog

When testing the system, I need to use and re-use my email address when creating test-blogs using the supporter plugin. But even when the blogs have been completely deleted from the admin interface it seems that the old email addresses are still saved - when creating a new blog using my email address it says it is still in use even though the blog has been deleted.

How do I change this? Does anyone have a hack so the emails are in fact deleted from the DB when a blog has been removed? It seems clumsy that you have to clear certain rows in the DB every time, if that is the case.

Thanks in advance for any help I can get...