This is just a suggestion, but in the case of fishers jumping in just to lay hands on the end game loot, why not just cripple the rewards for completion. That would eliminate the folks just jumping in for the swag. The only drag up would be the fact that a crippled group would be less likely for someone to jump in.

There is no good solution to the issues that I can see. But I am new to the who's mmo deal, so there isn't much stock in my opinion.