Another option, since it is in a closet. You could secure the servers so they can't be removed from the closet. Bolt two 2x4's horizontally so that they cant be pulled straight out, and bolt one going from the floor to the ceiling so the server cant be turned or anything.
(This works best with a small rackmount enclosure)
Better illustration:
(This works best with a small rackmount enclosure)
Better illustration: