1 point to Doug.

A stopper knot works well in this case.

"DSK" wrote

Aside from using a preventer or boom brake,
what is a simple way to ensure your boom doesn't
hit the shrouds in a hard gybe?

1- pull the main sheet in enough & cleat it (actually, your main sheet
should be cut a length such that the stopper knot hits the block just
before the boom hits the shrouds, but it's amazing how few people attend
to little details like this)
Fresh Breezes- Doug King