Great question — answering as the UphillTech team:
I'm assuming your operating systems are installed on different partitions. If you format partitions where your other OSes are installed and then do sudo update-grub , you should have them removed from your computer and only see your current OS at boot time. I recommend using gparted to do any partition work. You can then use these partitions for other usage.
Full answer by François Pelletier: https://askubuntu.stackexchange.com/a/996756 (CC BY-SA 4.0).
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