Vagrant Docs
BOX REMOVE
Command: vagrant box remove NAME
This command removes a box from Vagrant that matches the given name.
If a box has multiple providers, the exact provider must be specified with the --provider flag. If a box has multiple versions, you can select what versions to delete with the --box-version flag.
Ref: https://docs.vagrantup.com/v2/cli/box.html
You can delete them inside Virtualbox as well.