Yes! You can reject opportunities while personalizing. First, you must have Approval Workflow enabled. In the Personalize stage of the Workflow, you’ll see an option labelled 'Reject'. You are able to reject an opportunity for the following reasons: 'Couldn’t Find Data', 'Can’t Find Contact', 'Website Isn’t Relevant' or 'Just Delete'. When rejecting, it is good to write a note to let your colleagues know why it was previously rejected.
When you reject an opportunity, that opportunity will appear in your compose with the reason you rejected it.