Only knocks I tend to get are for pest control. Personally I look out my peephole, if I don't know the person then I don't answer the door and they go away, but even at that it's usually just the pest control solicitors and sometimes the kids selling for the school fundraisers.
On fliers, yes I agree the fliers are annoying. Unfortunately, I don't think most of the folks would listen to a sign and would just bypass it and put the stuff up or knock on the doors anyway. I have a neighbor that has a no soliciting sign on their front door and yet I see the pest control reps knocking on their door too, they tend to just ignore the signs.