How do you prevent friends from joining your game?

When trying to get some solo achievements or when running the game for the first time with a new friend, your other friends can be a bit too friendly and barge in, killing everything or just making the mobs beefier. How do you block people from joining your game?

Solution 1:

You can prevent people on your friend list from automatically joining your games by unchecking the "Allow Quick Join" option in the Social tab in Options (in the Esc menu), which is set by default.

If someone attempts to join your game as usual, it will notify you and ask your permission before they can hop in.