Buy a fish trap, or make one out of a 2 litter soda bottle. Put some of it's favorite food in it, and wait for it to go in it.
To make the bottle trap you will need an empty 2 litter bottle, a good length of fishing line, a small piece of life rock (this will help to keep the bottle sunk + it will give you something to rubber band a piece of sea weed or silverside to it).
Cut the bottle where the bottle starts to taper down.
Cut the top of the bottle to make a wide enough hole for the fish to fit through. (don't make it to wide just enough for the fish to fit through)
Invert the top into the bottle (secure it in at least to spots to the bottle with some fishing line).
Attach a piece of fishing line long enough to reach where you sit.
Once the fish goes in to the trap pull the trap straight up. Walla you just caught your fish.
Another way is to get the fish use to eating out of a net. Once it has become use to eating out of the net you'll be able to scoop it out.
Either method will work with greedy tangs, you just need to exercise some patience.