In Pocket Informant, you can create or delete calendars to meet your needs. To delete a calendar go to Settings->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.
iOS Based Calendars (the ones you can see in the devices stock calendar application and Informant) cannot be deleted from inside of Informant.
These calendars may be deleted from inside of the iOS calendar application in most cases. Some Exchange based calendars cannot be deleted even from the iOS application.