A rule that would prevent natural mobs from spawning inside of portal blocks.
It would still allow zigmen to spawn on random ticks in the overworld, since that is exclusive to portal blocks.

This is not an issue that can be wholly solved with "just don't use it" because portals can be used in farms without mobs spawning directly in the portal blocks (pushed into portals after spawning by water or pistons, for example), and in some cases those portals would also be able to spawn mobs (in the absence of this rule).