I had a couple of small crabs in my tank a while back. I got rid of them by putting some pieces of shrimp in my net, and holding the net in the water above the live rock. The crabs grabbed the net and went after the shrimp.
All I had to do was lift the net out of the tank - the crabs were clinging to it - and brought the crabs back down to the beach where I found the live rock they came off of. They just ran down the beach and into the ocean.
I live in Hawaii, and these were local crabs; all of my live rock was found after a bunch of storms washed a lot of stuff up on the beach a couple of years ago. You are allowed to take 1 gal
buckets of stuff off the beach at a time. Some of the rock I found actually had mushroom polyps on it, which later spread a bit through my tank.