From Console:

1

First Step

2

Second Step

Select your Server

3

Third Step

Make sure to Turn OFF the Server

4

Fourth Step

Under the General Section, Click the Files tab

5

Fifth Step

After that, Click on the server.properties file and go to the line that says allow-nether=true and change it to allow-nether=false and then click one Save Changes.

6

Sixth Step

After that, Head over to General tab, Click on Console and Start the server.

There you go! You have disabled the nether for your world!
How to enable/disable PvPHow to change Default MOTD of the Server