1. Go to Jobs -> My Jobs and click on the Manage button next to the job you want to cancel the monthly payment for.

2. Scroll down to Billing details, click on the Cancel renewal button (see screenshot) and confirm.

3. Your job will no longer be renewed, however you'll still get notified when it's almost expired and when it's expired - you'll always be able to re-extend or re-publish the job in the same job management page.