What is the procedure for cancelling my visa and work permit from abroad?

There is no "work visa" what you're asking about is a Non-B visa <- the B is business,

IF you were only on the initial 90 days of the original Non-B visa (as in you hadn't extended it at the immigration office) you would just be able to leave the country and it would 'cancel' when you left

However, with that being said;

IF you were on an extension of stay (that you got after the 90 day Non-B wound down from the immigration office where you work/stay) you should have cancelled it before you left the country by taking termination paperwork to the office that issued the extension and cancelling it officially. Remember just stamping out doesn't cancel an extension based on employment.

There's NOTHING you can do about either an 'open' (uncancelled) extension OR a work permit (which needs to be cancelled at the Ministry of Labour).

Your employer can cancel your work permit without you, BUT if you're on an extension they can't cancel that because the extension stamp is in your passport out of the country 😕

You just come back in, show the print out of the PDF they emailed you when the DTV was approved <- that IS your actual visa, and stamp in on that.

If you left on an extension of stay without cancelling it when you stamp in on the DTV you should expect to be questioned a bit about it. Just tell them you will go cancel it at the immigration office that issued it.

There's nothing you can do outside the country..