There's two methods that I tried.
The first is cutting a slit in a pool noodle and putting that over the fence. This worked for my neighbor's mare when she started windsucking. My horse, however, just pulled it off the fence.
The second method I tried, and that worked, was mixing Vaseline with chilly flakes (you can also your pepper) and smearing it all over the top rail. It was a bit messy in the yard for a while, but he stopped chewing the fence within the hour.
Hope this helps