If they are brown palys, you have to drive a silver stake throught their heart, otherwise they will grow back...
Seriously though, brown palys are notoriously hard to kill, you can try using an aptasia killer (aptasia-x or joes juice). They have worked on occasion, Ive killed a few recently but some strains will just bounce back from it. You can also manually cut them off the rock and glue/epoxy over the area and that should work as well.
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