This may be a bit elaborate but it works. It's called a Squirrel Stop. It uses batteries and when there is a sudden weight added to the feeder it spins, throwing the squirrel to the ground.
Here's a pic:
There are also several products that aren't nearly as funny that you add to the feeder pole that look like large drum cymbols that prevent squirrels from climbing up... hope this helps!
-Evan