Once everyone joins the party, put in the party chat what exactly are the rules.
If you make it clear that your group is a farm party, every player must have multiple clears, and this is not to get your first clear, then you should be set.

If someone does join for a clear, you report them to the GM. If you have evidence that your rules for the party were explicitly stated in the form of party chat, and the new player still refused to leave, then you have a case with the GM.

Or... you could just block every player who comes in for a clear. Never have to worry about them again.
Requiring an actual clear to use the party finder is a bit unnecessary. Besides, I've found plenty who know their role in the fight, but the rest of their party sucked, so they technically didn't have the clear. But perform optimally in farm parties.