You should change your browser settings to force-enable zooming on all sites.
Just track the paste operation and place a big red CHEATER banner on the username.
Also what can be disabled with javascript - can be enabled with javascript.
(I'm not really calling out HN here - its mobile experience is still better than most, although voting pretty much requires zoom)
The best way to do it was screw-to-zoom, where you draw a spiral to zoom in or out; righty-tighty to zoom in, lefty-loosey to zoom out. "Screw-to-zoom" is a bit gross for marketing, though. Maybe "spiral-in, spiral-out"?