When you longer need the OS image, you can delete the boxes in order to free up disk space. By the way, vagrant boxes are located in (a) ~/.vagrant.d/boxes on macOS and Linux and (b) %USERHOME%\.vagrant.d\boxes on Windows machines.

How to delete vagrant boxes. 

vagrant box list
vagrant box remove <name of the box>

Renaming a box. 

vagrant box repackage