Disabling the Nether
Disabling the Nether dimension is the same for all Minecraft Java Edition servers!
First Step: Stop your server and navigate to the File Manager.
Second Step: Open the file named server.properties, find the line where it says allow-nether=true and change it to allow-nether=false.
Third Step: Restart your server from the panel!
The Nether dimension should now be disabled for everyone in your server! If you want to re-enable it, follow the steps above but change allow-nether=false to allow-nether=true.
Disabling the End
Disabling the End dimension is only possible on Spigot/Paper Servers!
First Step: Stop your server and navigate to the File Manager.
Second Step: Open the file named bukkit.yml, find the line where it says allow-end: true and change it to allow-end: false.
Third Step: Restart your server from the panel!
The End dimension should now be disabled for everyone in your server! If you want to re-enable it, follow the steps above but change allow-end: false to allow-end: true.