mine out everything under their base except their floor, lace the bottom of the floor with TnT, wire a switch up to it - when they come home blow the floor - the combined explosion and fall might be enough to give them the hint.
There is no way to get around DeadBolt... However, if you are playing on a server with the capability of reporting griefers then do so,
and if your report include everything that was stolen. you have a 50/50 chance of getting your stuff back and people actually listening.