Frankly you didn't had to explain it, it's pretty obvious that some people only interested in the mount won't put any effort whatsoever in helping sprouts and probably leave when the duty involve slightly challenging/longer content.

I only took the mentor status to grind the mount, and never gave advice unless asked (or ask if they want some when they do obvious mistake over and over) never left a duty even when it's a clown fest as long as the party members communicate.

Now as i see it : If a mentor leave a mentor duty instantly or even before 10 min : the usual penalty + removing access to the mentor roulette for one week the first time, with increased penalty if they continue their behavior, and finaly removing of the status with no possibilities to get it on this account ever again.

At some point whatever your reasons are to be a mentor, you chose to be in a party of new players (or worse, notoriously obnoxious old players that are on an alt) so don't make their experience miserable just because you can't handle wasting time. You put yourself in this situation not them.