The OP isn't forcing anyone to do anything.. He even makes the rules of HIS party finder clear before starting. His method or rules don't need to be criticised. The point is people are joining, knowing the rules, and not following them.

It's exactly the same as stating 'no bonus' in a difficult fight, having someone with the bonus join, getting carried and leaving the party. If there are rules on the party finder then have some respect, stop being selfish and don't join if you're not willing to follow them.