I bought a bed extender from my dealer about a month ago. Upon purchasing and installing it I noticed that it is not at all difficult to become the property of someone else. So I pondered it for a while and came up with a deterent. It is not theft proof, but it may make someone move on to an easier target.
I purchased about 2 feet of thin cable (I don't remember the diameter) that has a rated strength of 200lbs. With a few clasps, I put loops at each end of each 1 foot piece. It is like a "Tether" of a sorts. I then bolted 1 end of 1 tether to the bolt that holds the bracket in place. The other end I removed a screw to the extender itself and put the other eye of the tether in place when I bolted it back in. The pictures show a better description.
A note to all, This tether system will be good for those who use there extenders like I do, for keeping the groceries in one place rather than having to pick them up from all over the bed. If you plan on (or currently do) taking the extender out and placing it back often, you may find this idea a bit to complicated for that type of use.