Is there a way to prevent people from joining your maps?

So I'm trying to get the "Complete X mission with S rating on X difficulty with <ally> as your only partner" quests. I create a new map, set rules to only allow Friends or Alliance members, and I'll get half way through map2 then BAM! someone drops in and my progress is cancelled out and I get no credit for the quest.

So one, is this a bug or is it intended to be like this?

You need to set a password and also click the "Also Limit Multiplayer..." option underneath the password field.