Paying a debt "in full"

If a creditor is sent a check marked “paid in full,” and he cashes it, how legally binding is this? Can he still demand the remainder of the debt?

nada, zip, zilch

Paid in full written by the debtor, now that’s a different thing.

Slightly contradictory evidence (in my mind, at least) from snopes. They say the notion is ridiculous, but then there’s this: