A VM is stored the same way as any other files on your computer. Simply delete the VM's folder (saved in "My Documents" on Windows hosts by default) when you are done with them. However, this might not be the wisest move. VMware and VirtualBox both have a "snapshot" feature so you can save the state of a VM at a particular point in time. I set up a "Base Install" snapshot for all my VM's so they can all be restored to just after initial configuration saving me the time and effort of setting them back up again. This is particularly helpful with the BT5 VM since running as root can have disastrous consequences in the event of a mistake.