Look into blocking portal creation where the portaller cannot build.

[LlmDl]

https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/event/world/PortalCreateEvent.html

[LlmDl]

So far this isn't looking to be possible. Initial attempt found that while the 2nd portal can be stopped from being made where it shouldn't be made, the portal-er will end up spawning on the second side regardless of the event being cancelled, usually inside of dirt.