A few incomplete thoughts:

Just let them expire and whoever wants to use them will (in theory) be able register them from. I.e., when you let them expire they will go back into the pool.

I say "in theory" because of the possibility that domain name registrars will "taste" them http://en.wikipedia.org/wiki/Domain_tasting and/or do something else to "grab" them and sell or auction them to the highest bidder in whatever marketplace they can expose the names to. Other downside is a domain name speculator will acquire and the the same. All this begs a debate on "right and wrong," which is a whole other topic.