In Pocket Informant, you can create or delete calendars to meet your needs. To delete a calendar go to Settings > Calendars, Triggers, and more > Manage Calendars and then swipe to the right on the calendar to delete:

Pocket Informant requires at least one event calendar to exist.  There are virtual calendars in Pocket Informant (Internal Task Calendar, for example).  Pocket Informant requires at least one non-virtual calendar for events.   

If you're unable to delete a calendar, insure you have at least one event calendar in your system.

Apple Calendars (The Calendars available in your Native Device) will have to be deleted within the Apple Calendar Application.