I understand your sentiment but, don't forget that in most cases it's not your co-workers or even your supervisor who makes this choice. I've worked one place where this was policy to prevent employees from doing Bad Thing(tm)
But if you haven't locked out the persons accounts BEFORE they state their intention to quit, you have already 100% failed at stopping Bad Things(tm).
Part of doing Bad Things(tm) is not to pull the action movie cliche of laying out your evil plans to the good guy before leaving him bound yet quite alive to escape and use that very knowledge against you.
You plan and execute your Bad Things(tm) while you still have the ability to do so, not after the time where there is an equal-to or greater than zero percent chance someone could prevent it.
Hypothetical - If you intended to murder someone, would you:
A) sneak up on them unexpectedly to assure the best chances of success? or
B) inform the target of your intentions so they can take steps to protect themselves and/or steps to prevent you from doing so?
True you may get lucky that the one doing Bad Things(tm) is stupid and does just that, but that isn't 100% either, and isn't something one should put themselves in a position to have to bet on either way.
The only sure fire way I am aware of to predict the future regarding when employment will be terminated is if it was initiated by the company.
You'll note in the case of terminations the above policy is almost always followed as a matter of course already.
But unless you can read minds, it's probably safe to say you won't accurately predict an employee leaving before they themselves decide that is an option and choose to act on it.
About the only exception is if the company is making that employees life so miserable already that quitting is the only logical choice - but even then I would argue that is the company initiating it, even if it is an indirect, round-about, and dickish way to do so.
Since you are way too late by this point to do anything to stop Bad Things(tm), why harm the professional relationship further? Treating them as a criminal can't possibly help you or the company in any way, so why cause a non-zero percent chance of harming you or the company by such actions?