Pump seawater in them. Any building in Gaza that has seawater running out of it should then be bombed immediately and the tunnels destroyed later. Depending on the quality of the cement, seawater will weaken it over time. The tunnels would then collapse.
There is some cement, but most of these tunnels are carved into the natural limestone.
We did the same thing to the Romans 2000 years ago. Worked for about 100 years.