Initial reaction: What? You don't like scholars? ;_;
Second reaction: How to get rid of Leeches... rename the spell to something else. Cleansing tonic, whatever.

In all seriousness, though, this has pretty much always been a problem =/ I think the suggestion I've personally liked the best was to have an option in PF, similar to all those "multiple runs for the loot", "serious play", "newcomers welcome" and all those checkboxes. Call it "clear required" or whatever, and have it check for an "instance cleared" flag for the listed instance. If you have cleared said run, PF allows you to join said listing. If you haven't, you get an error message, something like, "Unable to join party. Requirements not met."

Such an option would reduce the number of leeches in farm parties. Could it be abused? Certainly. Pretty much everything in this game can be abused in some way, if you want to. Would it eradicate leeches entirely? No. You always have those who managed to get carried at some point, or those who bought their clear run from someone. Another risk with this option is that the number of run sales would skyrocket, which in turn would promote RMT (cos those runs aren't exactly cheap).

The good thing with PF is that by joining a party via PF, you automatically agree to the terms listed for the party you join. Which means that if they break those terms (e.g. by not having cleared it when it's a farm party) I believe it's fair grounds for reporting them? At least last I heard, that was one acceptable reason for reporting people. The bad thing is that you can't know for certain until you actually join the instance, and it adds a lot more stress to the group as a whole, with finger-pointing every which way and time lost and potential food buffs ticking down and, you know. Stuff that's generally Not Good for a party and is likely to contribute to its disbandment.