When you say, "...if you first release the halyard..." do you mean you tried to douse the spin by releasing the tack line first?

On these boats you want to dump the halyard first and pull the snuff line in fast, until the tack comes tight then dump the tack line last and snuff the last 4 feet. You should not be going dead downwind when you do this or you may wrap your spin sheets around the snuffer hoop or even under the bow...don't ask me how I know. ;^) Idealy you want to be at about 45 degrees off dead downwind when you snuff, and faster is better.

When I'm racing Uni, I dump the main traveler down a bit so the boat won't round up, then I put down the tiller and use both hands to snuff as fast as possible, then return to driving and trimming the main. But tack line off last, always.