you need to find the source of the problem: too many nutrients - dead things, too much food, too much poop / high bioload. Unless you fix it, you will keep having problems. Adding more live rock gives more room for the good bacteria to break down the crap in your tank before it goes bad (I ain't no scientist, so I don't know all the technical terms).
You can get a good boost with chemi-clean, a powder that is reef safe the will totally annihilate your cyano in three days. The only problem is that cyano will eventually build up an immunity to it and it won't work anymore. No one knows how many times it takes to create the resistance, but it will work for sure one time. If you don't fix the excess nutrient problem, it will come back.
-Doc