System already set up. Linux mint 21 (if it is important, i can upgrade to 22). No disk or home encryption set up. And i don’t want to reinstall system, too many programs and too many custom settings.
Can you give a little more context re; “why” you want to do this?
Fundamentally you “can’t” prevent this from working, on any device, subject to encryption features and things like UEFI. Even if you stop the boot menu for example from allowing changes, typically you can still boot from a recovery device and do the same thing. Even if you lock down the BIOS, all someone needs to do is lift the lid and remove the motherboard battery to cause a BIOS reset.
What you can do is make it more difficult depending on the circumstances, but typically if you need to secure all the data on your machine the easy answer is “full disk encryption” … which won’t come with any sort of recovery routine, you lose the password and you’re done.