$ sudo apt-get remove i3-wm 

Uninstall i3-wm including dependent package

If you would like to remove i3-wm and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove i3-wm 

Use Purging i3-wm

If you use with purge options to i3-wm package all the configuration and dependent packages will be removed.

$ sudo apt-get purge i3-wm 

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 i3-wm 

See Also,