Mike Mason who blogs for Zentila had a few tips for dealing with attrition penalties.
You have a room block of 100 rooms but only 75 rooms get used for your event leaving you with an attrition of 25 rooms; rooms you have agreed to purchase from the hotel even though you have no one using them.
Mr. Mason suggests asking the hotel if you can purchase additional Food & Beverage items instead of buying empty rooms. This is a technique we employed several years ago when a client vastly over-estimated their room block needs (worth noting that they were using a different meeting planner that we replaced but not before the poorly written hotel contract was executed).
The client understood they were obligated to the money but by purchasing additional F&B in the form of bottles of soda and pre-packaged snacks instead of empty hotel rooms the client was able to use the refreshments for a different event several weeks thus recouping some of the expense of the attrition.
(Of course we had to work out the logistics of moving the inventory from our registration area at the downtown hotel to the client’s on-campus office but such is the life of the academic conference planner!)
Additional tips offered by Mr. Mason include applying the attrition to a future meeting or conference if the client hosts additional meetings in the same city.