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You can never use game skills on players. They can be used on PCs. That said, players choose the response of their characters. Those who value role-play will follow the skill result, while those who don't, won't. As a GM, I generally discourage PvP style play as a matter of play group cohesion, and have rarely seen players attempt to have PCs use social skills against other PCs other than in a somewhat humorous fashion, usually involving bluff.
No rules. General consensus is that unless magic is involved, players should always be able to choose their actions.
Bluff is opposed by Sense Motive to detect if the bluffer is lying, but PCs still have option to believe or disbelieve.
What I do is if a NPC would have otherwise rolled high enough to bluff, use diplomacy, or intimidate a PC, I frame the NPC description with that mindset.
Example:
Bluff not detected by Sense Motive:
"You get the feeling that what he is saying is honest. Do you choose to believe him?"
Diplomacy successful if PC was an NPC:
"What he just said was not only well-spoken but most people would say it makes a lot of sense. What do you think of it?"
Intimidate successful if PC was an NPC:
"This guy seems confident that he can mop the floor with you. You estimate he would be a tough to beat in a fight. Either way, if you don't do what he says, you get the impression you are going to have throw down with him. What do you want to do?"
A GM can use those social skills on a pc, but make sure to roll the pc's opposed rolls in secret. The result of a success will be that you have the npc say convincing things to the pc's - it's up to the players whether to be suspicious of the npc, and that is going to rely on your acting skills. If the players decide to distrust the npc despite a success, that is up to them, and they shouldn't know about the skill rolls in the first place. If the rolls fail, the GM should let the players know that there is something fishy about what the npc is trying to convince them of.
For a player vs. player situation, describe the feeling that a successful check gives the player being affected, but leave him free to react as he chooses. A successful Intimidate means that the other player character sounds scary, while a successful Bluff or Diplomacy means that the other player character sounds very sincere and honest. There are still many ways that a player character could react to such feelings -- for example, an intimidated player character might obey the other character, grovel, flee, or even attack if he feels like he has been backed into a corner.
With a little bit of creativity, the affected player character should be able to teach a player character who overuses Intimidate that doing so is a bad idea.
While I generally agree with and follow the philosophy that social skills don't overrule player control, I do allow two specific skill uses -- feinting with Bluff and demoralizing with Intimidate -- to be used against PCs as written. These two skill uses don't remove player control, they just offer particular mechanical benefits in combat.
Side note: As house rules, I apply a ranger's favored enemy bonus to Sense Motive to apply against feint attempts, and I apply any bonuses to saves against fear effects (such as a fighter's bravery) to the DC to demoralize, when and as those modifiers apply.