The way I finally got rid of the one in my last tank was to take out the piece of live rock the mantis lived in and flush it out with fresh water.
First you have to know what rock it lives in/spends the most time in. After that you need to keep an eye on it and figure out all the possible escape routes it may take--ideally the hole or tube it lives in will only have a couple of openings.
At this point you can formulate your plan of attack and set up the supplies you will need. You may have to move some sections of rock if they will impede the rapid removal of the rock with the mantis's tunnel.
Pre position the styrofoam container that your live rock came in, if you still have it, or if not a large bucket or (clean) plastic trash can by the tank to hold the rock once you remove it. At this point you need to regularly check your tank and wait for the right opportunity to extract the mantis's rock. Wait until the shrimp is just poking its head out of its hole, as it's more likely to try to retreat back into the hole than go swimming away from the rock while you reach in for it. Ensure that the escape routes are blocked (with a net, shell, etc., Not your hand!! ) and with one quick motion take the rock out of the tank and place it in the Styrofoam container/plastic bucket. Make sure that the mantis didn't get away when you pulled the rock out.
Take a moment and revel in the glory of having removed a troublesome pest from your tank. Then place a net under one of the exits to the mantis's hole and flush out the hole/tunnel with fresh water. The mantis will pop right out. If you have a mason jar you can fill it with salt water and keep the mantis there until you figure out what you want to do with it. I'd recommend keeping the jar in the container you used to hold the live rock after you put it back in the tank on the offhand chance the mantis tries to break out.