So when I play my single player games like HL2, I don't want steam to remind me of who's playing what multiplayer game at the moment. Is there a way to temporarily disable the reminders/pop-up's?
Open up your friends list, click "View" then "Settings".
In that window find "When a Friend joins a game" and untick "display a notification". There are a couple other notifications and things you can change there, so have a look at what else you may want removed.