blue tang is not hard to catch. remember, they sleep in holes, and go in the same hole when scared ! so when you put the net in water, most likely, it will run for a hole to stick its head into [a stag coral in my case] and then its easy to remove him.
putting food inside the net has worked for me in the past as well.
but the sure method, as mentioned, is to partition or take out LR ... but sometimes taking out LR can make the tank so messy, u cant even see the fish no more !