Sometimes when you try to delete a marketing user, the option is not available. This happens when the user has Admin privileges. Administrators can't be deleted, so to remove the user, you should first remove the Admin privilege.
Here's how:
- Go to Settings > Users
- Go to the Privilege tab
- Find the user you need to delete and de-select the Admin permission
- You will see a confirmation pop-up
- Go back to the Marketing Users tab
- Hover over the user you wish to delete, and click the red X
