If he's been a problem before, you can blind copy your response to HR. Be sure that his original emails are part of the reply/forward. Let work handle work. I work with some major PITA's. At the end of the work day, they no longer exist. A simple reply of "no thanks, it's not my color" should be neutral enough. He's making fun of bikes, not your family, religion, politics, race, etc. etc. Ride a couple of extra miles and dedicate them to him.