To kill is against I believe....... Even if its just a zooanthid.....
There will definitely be a competition for real estate, but whether that justifies killing the zoanthid is another question all together. The zoanthid will attempt to attack the sps by digesting tissue around the base with mesenterial filaments, followed by asexual budding onto any skeletal structure it has freed up for its clones. The rate at which this occurs can of course be slowed by ensuring the zoanthid is up stream from the sps.
The sps is aware of its chemical defenses being ineffective against that of the zooanthid, so it will not even bother entering into a chemical war. It will react by means of physical warfare, i.e. growth. If you provide the correct conditions for sps growth, sps will grow faster than the xooanthid can multiply. It will sacrifice basal encrustment (as it knows it will loose that fight) for vertical growth. Once it has grown to a sufficient height and aquired enough "mass" to ensure its survival, it will focus energy on physically combatting the zooanthid. It will grow branches above the zooanthid, attempting to overshadow it, effectively starving it from light needed for photosynthesis = energy = asexual reproduction. As it shadows itself and the zooanthids, the zoanthids should cease the attempt to encrust the upright stalk (as it is a rather uncomforatble position for zooanthids to grow, think orientation towards light source) and they will spread away from the encroaching shadow.......
So instead of worrying about who to kill, rather make the best of the situation and use their instinctive competitive behaviour to your advantage. Competition will lead to increased growth rate in both parties. Once the sps has grown to a height where you can comforatbly frag it, do so, as by that time you should have a small colony of zooanthids......