I would have done exactly what you did and ask them to pick up all their items daily. I have one family who leave their stroller here daily and it doesn't bother me but they wouldn't even consider leaving it here overnight to be in my way. That's simple consideration.

Other Mummy, your frustration and "End of the Rope" feelings are kind of evident in your post, best of luck in straightening out these people before you have to kick them to the curb.