“What do you do with a player(s) that max out their charisma based states and charm/diplomacy through everything you throw at them?”
An excellent question with a very easy answer! The great thing about roleplaying games is a number of rules that cover various issues and the ability to interpret rules in different ways for different situations. Much like the Judicial branch of American government, the DM has to interpret the rules put into place by the author to mold it into his or her’s story. These games are based on numbers and circumstances, all of which can be toggled with and adjusted.
Your question refers to someone who can smooth talk their way out of situations. As a rule of thumb, not every encounter is one that can be negotiated with. For example, those with low intelligence are probably going to be single minded enough to not want to hear what the ‘smooth talker’ has to say. An orc war party probably doesn’t want to hear the halfling babble on about how they shouldn’t kill them.
For a d20 based game such as Pathfinder, for example, you assign each level of difficulty with steps. A Minor difficulty would be a -2 penalty to a roll, where a Major difficulty could be a -10 penalty. On the reverse, you can set the Difficulty Check to a higher number based on the circumstances. If you deem a creature ‘mindless,’ then there is not a result that the smooth talking character can roll to persuade the creature. There comes a time where you have to really determine the circumstances of the encounter. If the smooth talking character is a known criminal by the organization who was sent to hunt him, then his words could be falling on deaf ears.
Does the opposition have coherent thoughts?
What does the opposition think of the character?
It is an option to let the person know that the opposition cannot be bargained with. There are some things in the world that would not be swayed by someone. Why would a powerful lich want to hear what offer the ‘nobody’ adventurer has to put on the table? Now, a more renowned adventurer could perhaps try to bargain since the lich would be able to see or even sense his or her power.
Remember, hindering a smooth talking character too much will reduce their enjoyment of the sessions. It is possible to meet them in the middle with what they want out of an encounter. Perhaps they are trying to win over a powerful minotaur who protects the maze. Maybe instead of joining their party, he agrees to lead them to the next floor if they reward him. It essentially becomes bartering. The player wants something. You can either give them exactly what he or she wants, or you can strike a bargain with them based on their rolls and roleplaying abilities.
Let me know if you found this method helpful! If you have a suggestion for another method, let me know!