Can anyone tell me what I have to do to cancel my work permit please?

Return it to the employment office where you got it. Get a receipt - just in case.

A teacher friend of mine recently completed his contract and decided to return to the U.S.He firstly had to leave LOS for a 1 month stamp to sort out his affairs.When they asked for his papers at imm,he kept smiling and pretending he did not know what they were talking about.Eventually they just stamped him out.When he leaves LOS,BKK will assume he left everything in the other office,I presume.

... better be careful, though. According to page 31 in my WP you're supposed to return your WP within 7 days of quitting the job, you got it for. Failing to do so, you'll: "be liable to fine not exceeding one thousand Baht".

Looks like it's best to return it! I'm UK bound on 9 May so I'll do it next week.