Yeah what he said, but go a step further if there are enough threads exposed to do this:
crank the nut on the clamp down. Then put a normal lock washer on and another nut over that. crank that thing down and bottom out the 2nd nut over the lockwasher & first nut. Then take another wrench and turn the first one backwards a 1/4 turn or so while holding the 2nd nut in place, that way they lock into eachother.