I'm pretty sure that's a counter to EVERY champion.
Well, hard CC'ing the enemy garen to stop him diving the backline is a lot easier than trying to get to the enemy backline and CC targets.
He get's passive armor and magic resistance, not counting any armor or magic resist gained from runes, masteries and items, plus a 6 second long 30% damage reduction PLUS a tenacity boost on a 11 second cooldown with 45% Cooldown Reduction. That's 5 seconds of time you can actually wound him. With a Spirit Visage and a Warmog's Armor, Garen only has to leave combat for 5 seconds and he's already at full health. I'd say it's pretty hard to stop a Garen.
It really isn't though. The reason people stop playing garen once they climb higher is that players get smarter and know what to do in situations. Focus and CC him down and he is useless. In bronze, people just don't know how they deal with champs like this.
Also why Yi and Yasuo are god tier bronze carry champions. People just don't know how to play against them.
You're bronze 1, basically very low in terms of player skill. While you may know what to do in certain situations (like not 1v1 these champions), they always manage to get fed off some other stupid teammate thinking they can fight 1v1. That's how league is.
Some champions are just god tier bronze champions like Yasuo, Yi and garen, but once you climb and you and your teammates start knowing and playing better, you will realize not having a gap closer/dash on a melee champion is a HUGE disadvantage.
But sure, now you're in the heart of low elo (and I know I'm still only gold), so if you feel like he is so unfair, pick him up yourself
stopping garen doesn't happen by one player. If your teammates don't know what they're doing, he can just get fed off them.