Suddenly I get QuickMatch games presented, but I cannot recall to have subscribed for such games. Will they keep popping up or is there a way to unsubscribe for them? I find the time limit too short and don't like to see my boot rate fly up.

From your browser click Multi-Player > Quickmatch and uncheck the "invite me to multi-day games".
From the app just click on Quickmatch and uncheck the box.

Multi-day Quickmatch is opt-in.

This should be a FAQ

There isn't anything in the Wiki about quick match at all. It should be added.

you could do it