I keep my nuclear green paly's in check by simply pulling them off the rocks with a pair of long tweezers when I do weekly water changes. I just have to make sure I leave nothing behind, or what is left behind will spout a new polyp.
Again a "beginner" coral that started as a small two head frag, which has blown up to well over 90 polyps within a few short months. If I knew how these "beginner" corals would spread like wildfire, I would have chosen something different.
I do keep them in check(Xenia, nuclear green palys, GSP, BSP), but its a huge PITA having to scrape them off the rocks almost weekly. On side note, and a small bonus to these nuisance "beginner" corals, I have a HUGE credit at my LFS from all the frags I take him. lol