Unless their sweepers are annoying neighboring corals, they don't really hurt anything. They're just unsightly. If you want to get rid of one, just crush it at the base. If you just break off the tube, it'll grow right back. You can also cover them with a dab of superglue gel.
Thank you. My Montipora digitata is the only one that being affected by them so far. Thanks for the tip about crushing them. I have cut back on broadcast feeding as well so I may see some die away.