I have has the Blue Cespitularia in my 32 gal Biocube for a couple of months now.
It's starting to really take off and I want to learn how to control it if that's possible.
The photo is from the LiveAquaria website - mine is fuller and the foot is starting to expand onto the rocks and sand.
It's hard to control most of the Xeniidae family. Culling is an easy way, if you can encourage it to grow on rubble (by placing the rubble accordingly), then you can just pull the rubble as it spreads (and either chuck it, give it away or sell it).
Also, make sure it is on its own island. Although I have had some break off the main colony and spread using the flow...
My hammer coral keeps it from spreading in my 220; it is neat to see, it looks like a force field around the hammer, keeping the, xenia in this case, about 5mm away...