Cyano relies on three things. Get rid of them all, and you can usually defeat it.
1. Phosphates. Come in from whatever you're feeding, or your water. Test your source water or have it tested to eliminate your water as a possible source. Cut down on feeding to reduce phosphate entering from the food. Get some type of phosphate remover (a couple good threads in progress right now on that topic).
2. Low flow. Cyano tends to grow in low flow locations in the tank. Put a PH closer to those locations, and that should help. Make sure you're turning your tank 10x.
3. Lighting. If you don't have corals, turn your lights off for a week, and make sure you're not getting sunlight on the tank.