I'm also in the process of fighting a hair algae problem, but I think I am finally winning the battle. I probably only have 20% of the algae that I had 2 weeks ago. As mogurnda said, you probably have to let it run its course, but there are probably some things that you can do to help quicken that course. These are the things that I've done:
1. I was running my MH lights 12 hours/day. I cut that down to 5 hours/day (I still have my actinics on for 13 hrs).
2. I "harvest" whatever algae I can every day. The stuff on the live rock I pull off with my hand; the stuff on the gravel I pick up with grabbers. Even though I can only get a fraction of the algae this way, I think every bit helps.
3. Even though my phosphate reading was 0, I added a phosphate reducer to my filter bags in the sump.
4. Every week I have been treating with Kent Marine SAT Biological Clarifier. This is a bacteria (not chemical) that competes for the same nutrients as the hair algae. It doesn't seem to affect my coralline algea, and it looks like I'm getting some other macroalgae growth, so it at least isn't doing any harm.
Much of the hair algae has been turning brown, at which point it's usually pretty easy to pick off the rock. Every day it seems to be getting a little bit better. Hopefully in another 2 weeks it will be all gone.
Good luck!