I've found that grabbing the paw and squeezing gently (not enough to hurt, but to squish their toes) while saying "NO FEET" works, nobody at my house is allowed to express themselves with their big feet. I had one adult rescue who was really bad about "punching" in your face, with him I had to resort to pinching him between his toes (you have to be quick and time it right).
Earnest knows a feet command, it's for playing soccer or other games with at big ball, we roll the ball and say "just feet" and he knows to knock the ball around with just his feet and not try to bite it.