Depending on the seriousness of your situation/what plants they are, you could just snip off the infected leaves, making sure you get all of it.
A blackout, however, is in my experience the easiest way to get rid of this stuff. 72 hours with no peaking, and all mine (admitedly on the substrate as supposed to the plants) had gone.
That doesn't mean to say that you shouldn't investigate the cause, though. It's all very well getting rid of it, but unless you work out why it's happened, it might just come back again.
Hope this helps,
Rob.