In my tank, I've noticed that GSP likes to climb. If it can find a connection to a rock that's higher in the tank, it's more likely to spread to that rock. To slow down the GSP's spread, I try not to stack other pieces of live rock on top of rocks with GSP. Anthelia is another rapidly-spreading soft coral that I try to keep near the top of my tank.
On the other hand, this trick doesn't slow down Kenya tree one bit. Rather than spreading by contact, Kenya tree spreads by "self-pruning" its own "branches". (self-fragging? autofragging?) The branches drift in the current, then grow wherever they land.
I don't have an anenome, but I've noticed that Euphyllias (frogspawns/hammers/anchors) seem to hold their own pretty well against GSP.