If you disable the Nether, players can still build nether portals, but the portal animation will play continuously without actually teleporting the player.
Log In to the Control Panel ↗
Select Your Server
Stop the Server
Click on the Files tab
Open the server.properties file
Set the Allow Nether Value
Scroll down until you find allow-nether=[value]. Change [value] to false to disable the Nether, or true to enable it. Click Save Content.
Start the Server
You have successfully toggled the Nether dimension on your server.