The two virtual machines you will be installing are configured as follows.
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If you've finished with your Rep Attack machines, you can easily delete them by shutting them down then right clicking on the machine in the VirtualBox Console and selecting Remove from the menu.
You will be asked if you want to delete the files containing the virtual machine. Select Delete all files. This will completely remove the machine and free up the disk space.
As long as you keep the original contents of the download or USB stick, you can always create the machines again in about 10 minutes!