On the pom pom xenia, it is quite easy to frag, one of the easiest and most forgiving corals to frag, although, thinbar (Xenia elongata) is more forgiving.
You have two options, both of which are effective. Many people who are growing xenia to trade grow it on a sloping grade constructed with reef rubble or small bits. The xenia will stretch and grow onto the bits, either separating entirely or leaving a small sort of bit attaching the two pieces. Should the xenia leave such an extension between the two pieces, you can slice it.
If you don't fancy having a bunch of reef rubble here and there, or, if you have a ton of xenia and need to just knock it out of an area before it bothers other corals, xenia is very easy to frag. Slimply cut or snip the coral at the base of a stalk to remove it from the mother colony and attach to a disc, plug, or piece of reef rubble. You can use many different soft coral attachment techniques. I prefer, if the base of the stalk is wide enough, to attach the xenia by piercing the base with a toothpick and rubberbanding the pick to a piece of reef rubble.
Xenia can grow quite out of control, but, whatever you do, don't be tempted to kill off a ton of it all at once. Once it gets bad, like it sounds like you've got a case of, it's going to be a constant watch to keep it under control. If there anyway you can make a "xenia island" by moving your xenia onto its own rock pile, far enough away from the main stack that it cannot reach and climb over?
