Please visit My tools > Waiting evaluations tab to view your pending signature requests. Click the ‘Disapprove’ option to express your disapproval for the evaluation and use the ‘Notes’ field to mention the reason for your disapproval.
Alternately, you can access the ‘Sign evaluation’ link from the email you received for this signature request, and use the ‘Disapprove’ button on the signature screen to express your disapproval.