OTP means one-time programmable. Original PROM chips may not be erased or reprogrammed under any circumstances.
The only "unsold" carts were in the hands of Gamestops and Walmarts, companies which would not have been able to do any such programming anyhow. Once SEGA decided to stop printing a game, the manufacturing plants stopped and they were left with none of said game in their possession for any sort of recycling to happen.