I'd probably use a 422 in that case. I think 422 errors are underused, and can good for communicating errors when a client requests something which doesn't make sense from a business context.
For those that haven't memorized every single HTTP error code, 422 is "Unprocessable Entity": 'the server understands the content type of the request entity, and the syntax of the request entity is correct, but it was unable to process the contained instructions.'
