When you bind a skin you have the option for maximum influences. the default I believe is 5, dropoff is 4 which are both fine to get going. Basically this means your verts will share 5 joints of influence with a 'smooth' of 4 which is about average.

the easiest way to do this without painting weights is to select the verts that are causing the probs, then open the component editor and find them under smooth skins. the verts in question will have values attached to them and will also be placed under the joints they are sharing. Merely set them to 0 under the joints you dont need them....

The same can be said for the rest of the skeleton. the component editor is useful for slight weight adjust too when you cant get specific verts to do as they are told.

cheers
Jay