A gelding who thinks he's a stallion

I've had Cowboy and Gem in stalls for several weeks. Cowboy is in because he doesn't respect the fence when there's spring grass. Gem is in because he leaves so many marks on the other horses. Here's the problem... mares are in spring heats and Rio, the only other gelding, thinks he's the stallion. I'm guessing he was cut late because he mounts the mares much more than Gem or Cowboy. He also has stallion behaviors all the time (low nickering to mares, territorial with the mares). Luckily, he's not at the top of the pecking order. So today, I let the boys out to get Rio in check. They don't have near the testosterone and they'll keep Rio from mounting the mares. His stallion-like behavior is becoming slightly dangerous for the people that come around.