I’d say ask a random teenager how would they approach the problem. They have malevolent group dynamics down to science. Mobbing may be illegal in school and at the workplace but mountain biking is a perfect safe space to practice. Eventually the smart subject would improve his/ hers behavior by employing a variety of sympathy gaining strategies or quit. Both may be a sign of intelligence. Dummies and weak units always quit under social pressure, after peforming a plaethora of self destructive acts, which strenghtens groups morale - it was us who were the righteous. But you started talking behind his back. That’s a progress for your group. I wish you luck, even though a group of so many people going for a ride together makes me cringe and I’d rather spend the rest of my mtb days being a lone wolf, riding IMBA approved trails on an E-bike with rear hub located motor aka Copenhagen hub.
Signed: a proffessional asshole.