The best defense against the seat in front of you is to request a bulkhead seat.
Anyone who thinks I'm being "inconsiderate" for reclining the seat that I purchased is smoking crack.... and if you touch me or kick my seat because you don't like me reclining, you're going to have a lot bigger problems than having reduced "knee space" (that's not meant to be a physical threat btw). If you're not happy with the space the airline provides you, then as Greg mentioned, buy a first class seat or fly a different airline.
And speaking of first, I was flying in first on CO
this past July, and had my seat reclined. The guy behind me, who had switched seats with his wife midway through the flight (she had been in the window, he in the aisle), insists on crossing his legs after moving behind me, so that his knee was pressed against the top back of my seat and the wall (ankle on the other leg). He then proceeded to kick/move my seat for the next 45 minutes. I was patient for awhile, but then realized he HAD to know what he was doing, so I turned around and asked him to stop. He then PUNCHED the back of my seat! This is a middle-aged, business-type. I angrily told him that was completely uncalled for and rude and called for the flight attendant, who after assessing the situation, asked me if I would mind raising my seat just a little because "the gentleman behind you is insisting on having has his legs crossed", and then asked the guy to please stop bumping my seat. I obliged, but I think the situation embarrassed the guy enough that he stopped acting like a 2-year old and didn't bump my seat again.
An unexamined life isn't worth living.