Drop an anemone onto the center of the colony. Lol.
I would personally shut off flow, pull as much of it out by hand as you can, scrape what is left stuck to the rock with a wooden bamboo skewer or too, or a metal one if you feel the metal will be safe for your tank, and then perform a water change. If any grows back at all, I would like to think it should be minimal and you could repeat the process until it stops coming back. Aptasia X or Joe's Juice should also work. I also agree putting an aggressive LPS near it would keep it under control, I would think. Just my 2 cents worth. Then again, i'm trying to grow my xenia... Lol go figure. I believe it depends on the kind though. Because I have light pink pulsing xenia that has barely grown at all since I got it and hasn't spread or broken off and attached in other spots, and then I have a brownish long xenia that is super long when fully open and it is growing fast and already spreading and attaching to my glass. I may get rid of it soon to make room for a big old rock of LPS or SPS when I switch to T5s. Plus I gotta get more rock in general in that tank so that I can place more corals in there.