My neighbor recently got out of the hobby and gave me a few books. In "The Marine Problem Solver" by Nick Dakin, here's the method for ridding your tank of a Mantis:
"Ridding a tank of mantis shrimps is not an easy matter. They are intelligent and can avoid traps set for them. If a shrimp settles in a favourite cave, you will have to remove the whole rock taking care that any back entrance is covered and that the creature does not slip out as the rock is lifted out of the water. If no other solution can be found then, as a last resort, you may have to push scissors, skewers, or a sharp stick into the cave to kill the shrimp by impaling of decapitating it."
Robb
"Ridding a tank of mantis shrimps is not an easy matter. They are intelligent and can avoid traps set for them. If a shrimp settles in a favourite cave, you will have to remove the whole rock taking care that any back entrance is covered and that the creature does not slip out as the rock is lifted out of the water. If no other solution can be found then, as a last resort, you may have to push scissors, skewers, or a sharp stick into the cave to kill the shrimp by impaling of decapitating it."
Robb