Good question.
Right implies some sort of permission given by some sort of guiding principle.
I guess the claim would be that since we are a legitimately democratically elected government we have the right to tell a dictator since he is just one guy and doesn't (probably) represent the will of his people.
Of course once someone has a nuke, you really can't tell them they can't have a nuke since they now have a deterrent whether you think you have the right or not to tell them.
The problem with the whole "right" thing is figuring out whose guiding principle should be guiding it. Sure you might get some Yankee who says we have the right. But who the hell is going to agree with that who isn't a Yankee?
Maybe a better way to think about it is Cost vs. Benefit coupled with what we Can and Can't do. In fact that is probably a good way to talk about ANYTHING a government does.
So what are the costs of Iran getting a nuke? To us? To the world? What are the benefits?
Once you get that, then ask yourself what can and can't we do about it?
Looking at it from that standpoint, I think it becomes clear that we should pretty much do whatever we can to keep those nutbags from getting nukes. Muslim extremists are like JW's with guns.