I got rid of huge clumps of prickly pears using a chemical...

I got rid of huge clumps of prickly pears using a chemical called Sprayseed that contains a mixture of paraquat
anf diquat.

You can't get small lots but if you know a farmer its likely they can give you some.

Round up does nothing to prickly pears because their waxy skin repels the spray.

But Sprayseed hits prickly pears like a nuclear bomb and a couple of sprays can wipe clumps 5 m high in a few weeks.