There is also the situations where a guy says he's going to college (or the pros or wherever) and that is that, you never get another chance to offer him a contract and the situations where a guy says "sorry I'm heading to college or pro football or whatever" however, when you click on the negotiation button, it still allows you to offer him his original demands. If that is the case, and you really want to sign him, offer him again his original demands and wait him out. he may be willing to sign at that point, or he may still opt to not sign. If he opts not to sign, offer him his original demands again, and play the waiting game. I once went 5 or 6 cycles thru that process before I finally got the #3 pick to agree to sign at a higher rate (11.0 mil+ bonus).
I believe (but could be wrong) the only guys who won't allow you to re-offer them their initial demands after their original rejections are guys who have their mother listed as their agent. If they have an outside agent, I believe you can always re-offer his initial demands and play out the cycle (which can take up to a week in real life) Given enough time, I believe any draft pick will eventually sign, its just a matter of the % odds of him signing finally falling in your favor.