juju, on things you don't want him to have, instead of offering it back when he gives it to you I'd offer him "something better" whether it be a treat or a special toy. I keep a couple toys that are reward toys, and Tommy only gets to play with them when he has done something particularly good like give up something very tempting. Perhaps you could keep some reward only chews or toys around to offer him in these situations when he gives up the rock or other undesirable object.