Hi,
Just stumbled on this issue today and found we have over 1500 ad hoc rooms completely took my eyes off this one. I have found a way to delete the ad hoc rooms though it is a bit long winded. I've found that if you go to all the sessions with the rogue ad hoc room and assign a sitewide room this then removes it from the list you can then go through all the sessions and delete the sitewide room which will then allow you to delete it from the rooms list. Long winded but works, i'm going to see if i can write a script to process these.
Hope that helps.
Thanks,
Naif