Bulk delete users has a glitch

I have a multi-user website and have lately been targeted by *evil* spammers who keep signing up. I managed to fend them off for a while with a good plugin but they seem to have found a loophole.

When I try to bulk delete them, there seems to be a glitch in the system which means that the top user of the page I’m trying to delete users from is also deleted.

In other words, if there are 10 users on the page and I’m trying to delete 5 of them using the bulk delete tool, WordPress will automatically take the user at the top of the page and try to delete 6. It means I can’t use the bulk delete tool reliably without living in fear of deleting my users and all their content (which they have paid good money to access).

Is this glitch known? Can it be fixed simply?