Block nether portals?

Last year we had a event with getting kids interested in architecture and instead of having to teach them to use a new software we decided to use minecraft, which many of them already know how to use and build stuff in.

We are doing the again this year, the setup is they are supposed to build houses in the city center of the city we live in. So we converted a GIS map to a minecraft world. Problem is most of them started doing other shit like build portals and fighting dragons.

Is there a command i can type to prevent them being able to build portals? We do the thing on a LAN server. i looked into using EssentialsX but i don’t know if that works with LAN or you need to create a server for that?

We just want to block nether and end portals.