I'm building an API for a url shortening service of sorts, and check when a POST request is made (to shorten a url) that the url is not a shortened url from my domain. If this is the case, I don't want to shorten it and want to send back the appropriate HTTP status code to say that the POST request is being denied. I was thinking that perhaps 403 Forbidden is the way to go, but I'm not sure if Forbidden applies to POST requests so much as GET.
Is Forbidden the way to go, or is there some other more appropriate code?