If you try the option “Block this application” on Facebook for the School of Wizardry, you may get an error screen:
Something went wrong. We’re working on getting this fixed as soon as we can. You may be able to try again.
Sure you can decline the invite, but you’ll just get more at some point in the future from the same friend.
I believe the problem is a FB Error, and to get around it you’ll have to Accept the Invitation and then click Leave Application.
That should clear up the problem.