Challenge him directly

“Actually,” you begin, and you can see his eyes light up with the thrill of debate, “the rulebook clearly states—”

“*Acktuuuually*,” he interrupts, “the rulebook was published before the 2.3 errata, which superseded the 1.8 FAQ, which modified the original designer’s intent as expressed in this podcast from 2017, minute marker 34:27, where he said—”

Six hours later, you’re both still arguing. The sun rises. Your voice is hoarse. He’s pulled out a printed binder of forum posts, cross-referenced and annotated.

You’ve aged ten years. He’s still going.

You collapse from exhaustion. The Codex Crypt Kreeper finally stops talking, looks around at the empty store, shrugs, and vanishes to haunt another location.