I've heard of people dipping their live rock in freshwater to kill of mantis shrimps and such but not sure how much damage it does to the live rock.
If you don't want to buy a trap you can make a simple trap by takeing a plastic pop bottle (like a 2 litre bottle) and cutting off the top.With the cap off invert the top half you cut off and push it into the bottom. Now you have a trap that the mantis can get in, but can't get out of very easily. Bait it with some food when your ready to turn off your lights and leave it over night. In the morning you may have a few mantis and some crabs but if it doesn't work at least it didn't cost you anything.
I don't have a picture but you want the part that was the bottom of the bottle to sit like it's on it's side and shove the part that was the top in it upsidedown so it looks like {< I hope that makes sense LOL:D attempting crude diagrams with symbols isn't my strong point.