playing campaign mode 

refers to playing the campaign mode of a video game (such as "Call of Duty" or "Halo") strictly to exclude someone who seeks to play online with others.

Can be expanded to describe any form of exclusion.

Richard: You guys are playing campaign mode just to exclude me!

Mike: No we're not!

Mike (in a private chat with another player): We totally are.

playing campaign mode by CoD Liver Oil June 15, 2009

playing campaign mode 

To exclude someone from a group social activity, as one might do by playing campaign mode

Toby: Hey should I invite Ricky to the bar?

Winston: Nah, tell him we're not doing anything.

Toby: I guess we'll be playing campaign mode tonight

*Both high-five*

playing campaign mode by Mothership Q June 16, 2009