I have a relevant example to this discussion. I'm running a one on one campaign of pathfinder with a friend of mine. in our last session she wanted to overthrow the leader of a local theives guild andtake it over. At level two. She wanted to roll to intimidate or persuade him to back down, or failing that challenge him to single combat. Again, she's only level two at this stage. So I told her outright that this charcter was way too high-level and she could not successfully intimidate or overcome him. She would have to do other things while she levelled up before being allowed to try that. So I'm with Rag, don't allow players to roll if you don't want them to be able to succeed at all. The same goes with failure, don't ask for a roll if you're not prepared for the players to lose. Sometimes I'll bend that rule though, and have them always get the necessary outcome but maybe do better or get a complication if they roll a crit one way or the other.
