How to make a computer impossible to access?

I know It’s not completely impossible to make it impossible to access, yet it would be extremely hard to do so. If you use all the basic settings and options such as setting up a bios or user account log in, it would still be able to be accessed by many means.

The only way I can see of preventing any and all sorts of attacks would be to create your own super file system encryption that would take hundreds of years to crack.

Anyone know of an easier or alternative means?