First, you will need to find the certificate on the website. To do this go to the "Jobs" tab, and look for the job which the draft Certificate is attached. Click on this job.
You will see something like the page below...
Click on the flame icon to the right of the draft you wish to delete.
This will take you to the draft of the certificate. Scroll to the bottom of the page, click on delete and confirm, as pictured below.
If you have done this correctly your draft certificate should now be deleted.
Once you have deleted a draft you cannot get it back! This includes if you turn an already issued certificate back into a draft by pressing "Edit". So make sure to be careful when deleting your draft certificates and only do so if you are certain you will never need it again!