any ol’ live media with gnome disk utilities (aka ‘disks’) or gparted would be enough to have an easy gui to clear the partitions. i usually use a linuxmint usb for that (and did use for that purpose a half-dozen times today).

it appears qubes uses anaconda (same as fedora) for its installer. there should be an option in it, where you pick your target disk, to ‘reclaim’ space. so op should be able to just do that during the (re)install process if they don’t care about the data on the disk.