Click the 'Sorry, we are unable to participate' button on the Request.
When you click the button at the top of the Request it will generate a notification to the client which says 'Thank you for your enquiry. Unfortunately we are unable to act on this matter.'
You can also send the client a message in the Comments channel. The private Comments channel will remain open until the Request is closed. Your conflict status can be changed by clicking the link Are you now able to participate? on the Request.