If the clown is currently the only fish in the tank, I'd use a trap (bought, borrowed, or homemade out of a plastic bottle) to catch it vice attempting to chase it around the tank. If you don't feed the tank and instead place food inside the trap, the clownfish should eventually swim into it to eat. Whether you decide to isolate it elsewhere while adding other fish, or take it back to a LFS at that point is up to you. I wouldn't worry about LFS taking it back -- they are used to it, though it won't be a full refund. Not sure what your LFS policy is, but many stores give you credit worth 50% of the price they are selling at.
