I've done some searching, can't find anything current, most topics discuss using pistons or tnt, but Etho does it easily in this video
He turns it on with a dispenser that has, I presume, a flint and steel in it, then turns it off just as easily. So, the question is, what's in the other dispenser that turns off the portal? He hasn't posted a how to video as far as I can tell.
Anyone know? The wiki says nothing about even turning a portal on with a dispenser, so it's a bit out of date.