Depending on youtr carrier, you can call them and have the phone "tirned off" - I had to do this when my son was using about 200 minutes a day when he was on vacation (the phone was supposed to be for emergency purposes only ) At least then they won't kill your monthly bill.

I would say that if someone disreputable picked it up that you'll never see the phone again. Have you tried calling the venue's lost and found?