What is the only reliable way to lower high cyanuric acid (CYA) in a pool?

Partial drain and refill is the only reliable way. Cyanuric acid doesn’t evaporate, filter out, or degrade with normal chlorine use, so water exchange is your only real option. Avoid expensive “CYA reducer” products—they work poorly. Drain a third to half the pool, refill with fresh water, and retest.