Do nba draftees have the right to say no to a team?

Well, they could in a way by not showing up or refusing to sign a contract, but they can't do forbid a team from drafting them if that's what you mean. But like I said, there are things they can threaten to do if the team actually does draft them. Look at Eli Manning and the Chargers. Different sport, but the same idea.

If they don't want to get drafted at all then yes. You can make it public during the combine and in interviews that you only want to play for a certain team and hope that the teams b4 that team don't draft you. But if a team drafts you and you don't want to play for them you won't get to play at all and you'll get a bad rep

they could but that decision will be costly

because that team has the rights for 1 year

also after that his value will drastically fall thus causing him a few Million $$$

yes.