When the requirements for mentors were changed it then reset their 'burger king crown' but wasn't exactly hard to complete the newer requirements. Changing or raising them again wont solve the issue.
Neither will having a report function or negative-feedback feature since it would be abused and not every sprout can be taught.
And with the suggestion earlier of having a renewal requirement such as running mentor roulette - again its not hard to do this. From my experience at least mentor roulette is no different to any other trial/dungeon/guildhest roulette as alot of the 'sprouts' are actually just alts.
A positive-feedback feature though could be something to add. Similar to the player commendations, players could tick a mentor as being helpful. Maybe 1000 'ticks' gets you a title or glamorable-crown.

One thing I want to note here though is that just having mentor status doesnt mean it should stop being fun for us. I was guilted this morning for trying to buy a personal housing plot just because i was a mentor and therefore i should be leaving it for a sprout? Thats just not cool bro.