To cancel your JobMail, sign in to your account and click your name at the top right of the page. Now click Saved Searches. From here, un-tick Email me new jobs next to the Saved Search you wish to turn off. You can also select Delete to remove both the JobMail email and saved search.
Alternatively, to unsubscribe instantly from all SEEK JobMail notifications, open the most recent JobMail you've received, scroll to the end of the email and click the Unsubscribe link.