I had a similar battle with one of those very same crabs.
He was blocking up the holes in the rock where my Firefish slept at night. She kept trying to get in, but ultimatly she was out in the "cold" for a couple of days.
Figureing he was becoming a bit to agressive and may even try Killing her, I knew something needed to be done.
Unfortunatly I don't have a tank with a Mantis to exercise, So I ended up dumping the crab in the toilet.
(I wanted to just kill it quickly by smashing it with a hammer, but my fiancee said that would be cruel. She wanted to flush it.
Hardware: 37 gallon SW
tank and stand by Oceanic Systems, Magnum 350 Pro Canister Filter (half carbon), CPR Bak Pak 2R skimmer, Maxijet 1200 Powerhead, RenaCal 150 W heater, Compact fluorescent lights with moonlights. 40 lbs of liverock, 3" sand bed.
Software:2 Tank-Raised Ocellaris Clowns, 1 Purple Firefish, 1 Electric Orange hermit crab, 18 Blue Legged hermit crabs, 8 or so Nassarius snails, Xenia, Blue, green, and Green Striped Mushrooms.
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Central Illinois Marane Aquarist (CIMA) member.