How about having a lock thread button that works in the similar way as the expired button ie if enough people click on it, then the thread is locked. This would be limited to Expired deals only.
I guess I'm thinking of occassions when a deal has expired but for one reason or another (usually not very good reasons) rambles on for another couple of hundred posts. Sometimes when this happens mods will eventually lock the thread and create one in misc forum. If the members could do this themselves then its one less thing for them to do.
The PS3/empire stores thread is a good example. Its cluttering the deal forum and has no real purpose being there now.
If the deal is expired and locked but becomes unexpired then we would do as we do now and ask a mod to unexpire.
I'm sure there may be good reasons for not locking threads but its just a thought.......