"Let the wookie win."
If a player hits on this idea, they'll keep pushing the limits until they figure out ways to effectively kill monsters using polymorph. You can go back and forth in a somewhat antagonist game of cat and mouse with the player where you explain away why this and that approach doesn't turn polymorph into an effective instant kill ... or you can just let it happen. I find it far better for the game to work with the PC and make it fun for them. To that end, I join their game rather than trying to end it.
Player: "So the Grick fails and turns into a starfish? I scoop it into that glass jar from the last room and put the lid on!"
DM: "Roll me an Intelligence check - proficient if you have Nature."
Player: "13."
DM: "The starfish will die in a matter of minutes without water. That will end the polymorph and return them to their normal form at full hp. That'll break the jar, dealing a little damage to them, but they'll essentially be back at full hp then."
Player: "Oh - what if I put water in the jar?"
DM: "If it isn't salt water, the starfish will die relatively quickly still. You're not sure how long, but it will be less than an hour."
Player: "OK. How long would it take for me to get back to that room with lava?"
DM: "You have enough time assuming nothing new has moved into your path."
Player: "OK, I run back and throw the jar in the lava."
DM: "You rolled a 13? As you start to head that way you realize the grick may survive a dip in lava - it'll likely be badly burned, but it is likely to survive. You realize, however, that you could just place the grick in a confined space. If the space is strong enough, the grick will be crushed to death when it reverts."
Player: "Oh - there was that stone shaft that we pulled the key out of ... you said that was drilled about 10 feet into the solid rock."
DM: "Yup. You think that will work."
Player: "I do it then. What happens when I end concentration?"
DM: "There is a very brief screech followed by a surprisingly thick red and green goo with specks of bone extruding from the mouth of the shaft."
Player: "Ewww ... nice."