traps maybe. Take an olive jar and lean it against a rock at an angle and put some smelly food at the bottom. Make it a relatively deep jar. When mr crab goes in to food he should slide to bottom and they have issues climbing up that surface if its at a good incline. Also try dropping food at lights out a ways from his den, leave a net in the tank prior to lights off and he should go for it. Try and get it out on the sand a ways from his den then scoop him up with net. If you know the rock and can get it out easily you can try soda water, high saline or fresh water to chase him out. I would get all of them out and not chance it. Crabs are the ultimate oppostunists and even 'safe' crabs like emeralds, sally lightfoots can be real terrors on other fish and inverst when they get bigger. Any hairy crabs are generally bad news. I keep all my hitchhiker crabs and unwanted ones the LFS gives me from LR in my curing tank an 18 gallon tall I use for curing live rock. Currently housing my extra large and troublesome emerald, a brown gorilla crab, one large hermit I caught pulling up my zoos and another giant hermit / decorator covered in anemones. I refer to this tank as the 'penalty box' where nasty critters get to have a time out. If you manage to catch them and no one else will take them let me know rather than flushing them I find em pretty entertaining.

One of these days' I'm tempted to let my 6" Peacock mantis loose in there and see how long the little troublemakers last