If you want to reject a job, because it doesn't conform to your requirements, begin by clicking Jobs to Accept:

You'll see all jobs which are at Status 081 (production-ready) and haven't yet been accepted. You can sort the list by pressing the column headers, or filter by Production Method if you've set up more than one.

Rejecting a Job

Click the tick box in the job row and hit the Reject job(s) button. That'll put the job to status 091 and notify the workgroup that placed the order.