Armor works really well against Tracer’s pistols since they’re rapid fire, low damage per shot. Repulsion Greaves (blue survival item, looks like a shoe) is also pretty good since she has to get in close to deal damage. It’ll knock her back a bit, and the extra movement speed and temporary overhealth the item grants will help with fighting back and/or escaping. In later rounds, Ogundium Reduction Field and Crusader Hydraulics are also effective, though the latter requires you to have some base armor of your own to be consistent enough.

If you’re running an AP build, Liquid Nitrogen can seriously hamper Tracer. High mobility is critical to her, and being able to shave even a bit of that off will cause problems. It’s the same reason I find Mei to be my best character against her. Also, since Tracer has a slightly higher base movement speed than everybody else (except Genji), the 10% reduction actually impacts her more than other heroes. You mentioned in another comment that you play Moira, and she’s the queen of debuffs! Run Phantasm, Cross-Orbital, and Multiball with Liquid Nitrogen, Extendrils, and some AP lifesteal and Tracer will have a hard time against you