You could certainly do them one at a time, search for banned members then click on the red x on each one of them.
I would guess you are looking for a more elegant method, which would most probably be an data base query. Rather than I taking an educated guess what the exact query might be I will let someone else give that to you which actually knows.
There is no simple query for this. There are too many tables involved. You would need a proper addon to process the users through xenForo's user system. If you are a programmer then you can post in the Development Questions forum for help with this. Or post an Add-on Request and some one may be able to write this for you.
I am sure this feature will be added in a future version. But as the devs have said, they had to pare down the feature set for 1.0 in order to release it in a reasonable time frame.