If you're looking to delete an account that has been set to be an administrator, the option to delete that account will only be displayed if the user is changed to be a team member or a client as admin users tend to be important users and we want to ensure they are not delete by mistake.
For help doing this, please see the following pages:
http://support.sendible.com/entries/82847659-How-Can-I-Edit-A-User
http://support.sendible.com/entries/83198605-How-Can-I-Deactivate-Or-Delete-A-User-