Xbox and PC players of Call of Duty: Modern Warfare II are unable to turn off crossplay, a feature that allows players from different platforms to play together.
Only PlayStation players have the option to disable crossplay, which can be done in the settings menu.
There are several reasons why players might want to disable crossplay, including the prevalence of hacking on PC and the advantages of mouse and keyboard input over controllers.
PC players might also want to avoid controller players, and the ample aim assists they benefit from.
Author's summary: Xbox and PC players are forced into crossplay in Call of Duty: Modern Warfare II.