I have a player that is claiming "that's what my guy would do", but I don't think they're actually suffering My Guy Syndrome.
Based on What is "my guy syndrome" and how do I handle it?, a My Guy Syndrome player uses character attributes, past roleplaying and what-have-you to justify certain behavior in-game, thus disclaiming his decision-making. The character will do what he's meant to do, no matter the outcome, because "that's what my guy would do".
In the accepted answer, it is clear that the character itself actually could follow that rationale were he an independent entity, so much that all players end up agreeing "that's what that guy would do". This happens because the choices make sense according to information on the character sheet, character class/role, past choices, etc.
But I have a player who claims "that's what my guy would do" when in fact everything denies that. Very basic case: a Lawful D&D player-character persistently roleplayed in a very Chaotic manner, yet the player claims "that's what my guy would do" (i.e., "he has to take advantage of others and lie about it, no sane character would do different"). So there you have it, that's not what his guy would do, but he says we have to put up with it.
In this particular real-game situation, the player obviously decided he wasn't profitting maximally by being Lawful, so he just decided taking Chaotic steps when he felt that was convenient.
Personally I understand why people use the "My guy would do" line — I've done it before. In fact, as the example in the other question goes, all the other players can end up agreeing with the My Guy Syndrome player, even if it could ruin a game. But making up as you go whatever "your guy would do" can be doubly frustrating to players, not to mention the GM who built a game perhaps even anticipating "what the guys would do", only to be undone by decisions that are very much out of character.
My question is, how do I deal with a player like this, preferably in a way that avoids frustration of both the problem-player and the other players?