I know people who go to the big university where I live who have obtained torx screws with the same length, diameter, and thread pitch (this is important because a different threads-per-inch will strip things) and replaced the Allen head bolts with Torx ones. Torx screwdrivers are easily available, but this isn't a tool that a fork-jacker would be carrying on their person. There are other methods people have used with varying degrees of success, from using a low melting point wax and a BB, to using Loctite Threadlocker Red (which needs either a *lot* of force or heating the bolt to 300 degrees Farenheit). Trick is... it's quite easy to make something impossible not just for thieves, but for the owner to remove something, so high strength threadlocker is pretty much out of the question. Instead, I'd go with something more proven to keep bolts in place, either by replacing with Torx, Pinhead or Pitlock, or if one has deep pockets, hitting Bryce Fastener for screws with a custom keyed head that only you have the screwdriver bit to.