Probably everybody knows them, Skype notifications on the bottom right: “xxyy is online“.
I find them pretty annoying if I’m working on something. You can press an “x” to hide the notification, but to turn them off you have to go to “settings”.
Why not create the possibility for users to turn the notifications off directly from the notification? Eg. when you hover over the notification, display a small text “turn off”. Don’t make them start the program, search for settings, search in settings the “notifications” setting etc. I’m to lazy for that and I bet you are too.