I think there already is a built in disincentive to play 0 or low-ranked players. A win adds very few points but a loss results in a huge point degradation. If Stone wants to play only low ranked players, who cares...whatever floats one's boat.
FoD COULD look at implementing a point differential limit, but that might punish the elite players who are in the 500+ level - they would have no one left to play. If you WERE going to implement some sort of restriction I think you would have to take the number of matches played into consideration as well. A 100-point player with a lot of experience is often capable of knocking off someone at the 400 level.
I like to use the ranking and dot-accumulation to try to pick an opponent that will give me an even match - the best games are the ones that are decided on the final stone, either way. Of course, after a while on here you find people that you enjoy playing...for me, those are the ones who combine sportsmanship and reasonable skill.
My main regret on this site is my lack of ability to understand other languages...I am probably missing out on some great friendships here because of that.