It isn't a question of "rules." It's a question of stupidity. If conservatives will not draw the line that says, "We're not voting Republican if Giuliani is the nominee," and mean it, the net result will be that the political establishment will not take conservatives seriously, and will go right ahead and nominate Giuliani anyway. Count on it.
Honestly, I think Rudy will be a disappointment to those who think he’d be better than a dem. I wouldn’t doubt that he’s trying to come off as conservative as he can to win the Republican nomination. I shudder to think what he’ll do if there’s no more motivation for him to do that. He may just abandon all pretense of conservatism.