Hi Tabitha,

I know this is a very late answer, so you probably already solved the problem.

But for next time...

Create a new SYSTEM role called  "BANNED" or something:

For seminar signup capability set it to "FORBIDDEN".

Assign user to role banned.

Now user is incapable of signing up on any seminar in the whole system.

I saw this trick about forums where the goal was to stop single user to enter new or reply to messages in forums..