Rear wheel keeps moving forward in frame no matter how hard I tighten the bolts. Any solutions? This is occurring on my Avanti Discovery 8, which is a hybrid bike with an in-hub 8 speed.

Problem is I seem to be applying so much torque it's pulling the rear wheel forward in the frame, loosening the chain. I've tightened the rear nuts as hard as I can - to the point where the nuts got a little damaged (hex edges started to 'flatten' inside the spanner) - but the wheel keeps moving forward as soon as I go up a hill and apply pressure. You can even see the scrape marks where the nut and washer has been digging into the frame and being pulled forward.

Is there any solution to stop this?
Take a look at something like this:

http://surlybikes.com/parts/small_parts/monkey_nuts

Edit: this may me more along the lines of what you're looking for

http://surlybikes.com/parts/small_parts/tuggnut

I'm sure other brands make similar things.