@Jeff - Umm, I'm not actually sure now. I just tried replicating what I did last time but it didn't work!
The only way I find of doing it is by deleting all authorised users and then add the one you want back in again. To do that search for "twp-authed-users" in your database (in the wp_options) table and delete the contents of that cell. It'll start something like
Once you've deleted that content then the authorised twitter users will be blank again and you can just add it back.