I’d fill them with oil and light them up. Make them scared of going into a tunnel.
Look on the bright side. The money we send over for them to build schools is being used to build tunnels and using kids to build them. This gives the kids a bright future in coal mining. Whoops, they are closing the coal mines.
I like the idea of the snipers/claymores, but I think firing the hole would do one extra step for ‘free’: It would showcase the other ‘rat holes’ that are inside the perimeter, as the fire/smoke would waffle up any openings along the full route.
Just make sure the fire patrol doesn’t let it spread BEYOND...