Do you want to get rid of all of them or just a few?
If you want to get rid of all of them you can simply drop the contents of the ost_users table (note: I'm not at my installation to check but I think that there is also a user_emails table or something similar that you would also want to nuke.)
If you want to get rid of select ones then you can use the Uses tab, select them, and under More (gear) there should be a delete option.