tried the enderpearl thing, its kind of neat i suppose but combine it with the "w" key and you can go anywhere you want in the current version of tekkit. until they update i would just hide butt-loads of chests, or personal safes =D, at various heights with butt-tons of cobble, dirt, whatever to make their lives a bleeding nightmare and hope they don't find the one with the bootie. on a side note, i don't know if its just my texture pack but the black hole chests have some alpha blending issues that make them visible through blocks at distances of about 15m+. leads me to believe your x-rayers are just modding their texpacks to do the same for items they are looking for
of course all this talk about greifing makes me think of the old saying "if you can't beat them, join them", of course in this case i mean make your own server and boot the hackers, not become one.