$ sudo apt-get remove ns3 

Uninstall ns3 including dependent package

If you would like to remove ns3 and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove ns3 

Use Purging ns3

If you use with purge options to ns3 package all the configuration and dependent packages will be removed.

$ sudo apt-get purge ns3 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove ns3 

See Also,