/facepalm just punch the glowing purple part inside of the frame and relight it with flint and steel any time you want to go to the nether... And what's with that guy who said to just mine out the obsidian, it'd take a hell of a lot of time each time you do it.
You could always have a small pool behind the frame that barely touches it, and have piston that pushes a block to cut off the water, that is connected to a switch that also activates the dispenser to light it. And when you turn the switch off the water should flow back onto the frame turning it off.
Disable: set up a dispenser facing one of the portal blocks, put a water bucket in it, and connect it to a quick two-pulse circuit. When you activate the circuit, the water bucket will be poured and break the portal, then quickly be sucked back in by the dispenser.
Enable: set up another dispenser at ground level facing the portal, and put a bunch of flint and steel in it. When this dispenser is powered it will light the portal again.