Rules lawyer

A rules lawyer is a participant in a rules-based environment who attempts to use the letter of the law without reference to the spirit, usually in order to gain an advantage within that environment. The term is commonly used in wargaming and role playing game communities, often pejoratively, as the "rules lawyer" is seen as an impediment to moving the game forward. The habit of players to argue in a legal fashion over rule implementation was noted early on in the history of Dungeons & Dragons. Rules lawyers are one of the "player styles" covered in Dungeon Master for Dummies. The rules of the game Munchkin include various parodies of rules lawyer behavior.

...
Wikipedia