I need a very secure deadbolt for an outbuilding. The antibump locks that I've seen are for homes?

Strong hasp and padlock with a number code, and don't give anyone else the code. On that kind of lock, the kind with little dials so there are four numbers, you can change the code so if you ever have to give the code to someone, you can change it the next day. You should be able to buy one of those anyhwere that sells locks. Or replace the existing deadbolt and put the key somewhere secure where no one is likely to find it. You can buy a deadbolt lock at Wal-mart for $15. No lock is completely secure if someone is willing to destroy things to get in, but the most secure lock in the world does you no good if you let the keys get into the wrong hands.

You can either install a new lock, or replace the tumbler (far cheaper). Most deadlocks allow this and as long as you can open the lock, it is rather easy to do. If you can't open the lock because the keys are gone, call a locksmith to open it. This would be cheaper than prying it off and potentially damaging the jamb of the door, which is a bigger deal than one thinks to fix.

Then, keep they keys on your body, or hidden somewhere well. Also, if the family member does not live in your home and you do not want to press criminal charges, get a cease-and-desist order prohibiting him/her from the property.