The topic will explain as to when it is possible to delete a job and how to do it:

This will show you how to delete a job.
There will be times when you may need to delete a job. It can happen if you realize that one job is a duplicate of another, or a job is created, but the customer declined to accept your company’s services because of various reasons.
QuickBooks allows you to delete a job when possible. However, please be advised that you can only delete a job, or customer, if there is no transaction associated with the job or customer. If you allocate costs to a job, or if you create an invoice for a customer, then you will not be able to delete a job or customer.