Im still fighting a little but have made some really good progress since I returned from vacation(take great care in who you allow to feed your reef). Here are some things I did that have helped.
-Identify what is causing it to be there in the first place. Overfeeding, low flow, etc. Then correct the problem if you identify it.
-Use some phosguard or the like for up to 4 days, test then use more if needed. Be sure to rinse it before use because the dust is harmful to corals.
-Do some water changes and use your siphon as a vaccuum(sp?) and try to get as much of it out as possible. Be persistent in this step because if you kill it off with one of the above steps, the die off will be fuel for new cyano, so it needs to be removed.
-Cutting your photoperiod back will help kill the remaining cyano and will have minimal impact on corals if the lighting cycle is down to just 3-4 hours for a couple of days. You dont always get full sun in the ocean.
Hopefully this will help you and remember that everything in this hobby takes a great deal of time, so dont expect some overnight miracle. Good luck!