How can I get my dog to stop eating off the ground!!!?

Hes 6 months old and is a pomeranian. Everytime we go on a walk he has to pick up rocks, pebbles, sticks, clumps of gr, dirt, whatever you can find and its starting to piss me off because as soon as I force him to drop one thing hes somewhere picking up something else. I say no sternly and he doesnt listen its starting to effing piss me off..