I prefer cement.
I met a kid from my alma mater that was from Israel and studying mining. He said in Israel he was a demolitions expert and would blow up the tunnels. “So I know how to destroy them - now I’m going to school to learn how to build them!”
He said that when they found the tunnels they would fill them with crap from the septic system trucks first - then they would blow them up. Often the enemy would have kids and women in the tunnels working - so they would flush them out first.