Is there a way to shut down a Nether portal with redstone?

You can put a single bucket of water into a dispenser. When a dispenser "dispenses" an empty bucket, it actually "picks up" a source block in front of it. By putting a single water bucket in a dispenser, when it's activated once it dispenses the water and when it's activated again it removes the water.

You can use this to make a button- or other restone device-triggered "off switch" for a portal. This is a complete reference design:

The trench behind the portal is what keeps the water from flowing everywhere:

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To turn it back on, you can build a dispenser with Fire Charges directly against the frame. The Fire Charge goes through the frame, lighting it. The only problem with this is that it consumes a resource that has to be occasionally replenished. Thanks to Resorath for the tip and the video that demonstrates the technique:

I happened to be in need of shutting a portal down from the Nether side.

I built my project in an Overworld test world and when i finally built it i realized i couldn't shut the portal off because i was using a dispenser producing a single tick of water, which unfortunately, doesn't work within the Nether!

I happened to find the simplest of the solutions, that im writing down here just to add to the previous ones. LAVA

By dispensing lava into a open portal, we break it. Nether side fixed. Yeah.

Here is my portal on/off setup, as compact as possible.

This is the front, in the left dispenser i put Flint & Steel, in the right one bucket of water (if Overworld side) or bucket of lava (if Nether side)

Hope is useful!

Minecraft version 1.5 allows flint and steel to be used in a dispenser. When the dispenser is activated it will light the Nether portal.