This specification defines the QUERY method for HTTP. A QUERY requests that the request target process the enclosed content in a safe and idempotent manner and then respond with the result of that processing. This is similar to POST requests, but QUERY requests can be automatically repeated or restarted without concern for partial state changes.
I believe you can use 422: Unprocessable Content for this.
“I know what you said, but I’m not answering you.”