What Is the Process for Cancelling an ED Visa in Thailand?

And we're done.

To re-cap, you cannot show up at the border with a letter stating you're not gonna study any more and have your ED visa extension cancelled.

IF you are going to try to bounce out and back by LAND to get a new 30 day visa exempt entry you need to GO to the immigration office you got your extension from with a letter from your school stating you will stop studying on xxx date (most offices let you go a few weeks early, although some don't). The office will cancel your current extension and give you a new stamp stating you can stay until the date on the termination letter states. THEN you can just leave the country when you want to (on or before that date obviously).

In October on extension of stay on “Ed Visa”, I brought the letter from the school to the Mai Sai border and they sent me to immigration to cancel it first.

School should sort that for you. At least paperwork for it

As a rule you can take that letter to the immigration office a week or so before your actual last study date (the date the letter says you're stopping)

The immigration office will cancel your current extension and give you a new stamp dated until that date. Then you just leave the country on or by that date..

A visa does not and cannot be cancelled

An Extension of Stay can and should be cancelled at the immigration office

Id speak to the school/uni that gave you the ED visa as a lot has changed recently they should be able to give you some info.

You need some sort of letter from your school.