@Andy *****
, this image shows you the difference between a visa and an extension.
The two images at the top are visas gotten from a thai consulate outside the country and the bottom left image is a visa gotten from the immigration office inside the country. If you quit work, and canceled your work permit none of those would cancel and they would be valid for stays until they expire.
The bottom right image is an extension of stay, <- that is the one you cancel when you cancel your work permit.