For many cameras, there are locks or lock boxes available. Google for "lock trail camera", and you will find plenty of offers.
fix the camera in a tree high enough you need a ladder to reach it, so people cannot just "accidentally" take the cam. They would have to bring a ladder as well, I guess most thieves won't invest that effort.
use a cheap cam which is not worth the hassle (and in case it gets stolen, your financial loss is small)
install them only in a place and direction where it is more likely to take just pictures of animals, not people. If one tries to use it for catching people doing illegal or antisocial things (like described in this question), then those people may feel encouraged to remove the cam.
Of course, there is no 100% guarantee your camera won't get stolen either, but I guess more drastic measures like a burglar alarm or a GPS tracker are quite impracticle to use and install, and probably won't be worth the extra money.