I think you can re-enter the draft the following year in hopes of getting picked by another team. So if a player says I'm not playing for you, most teams probably don't want to waste a pick on a player they'll never get.

For players there is no benefit to being drafted early in second round vs going undeafted. Only first round picks are subject to the rookie scale, everyone else can get paid as much or little as they can negotiate. So theoretically the last pick in the draft or someone undrafted all together can get paid more than the first pick.