2 part epoxy that you knead together to activate.
People make a little strip out of it and make almost a perimeter where you don't want the monti to grow. Once it encrusts over the hardened epoxy a few months down the road, you peel the epoxy back and the monti comes with you. You now have an awesome frag to sell, and the monti has to start its invasive manoeuvres all over again