In the Calendar app, tap the Calendars button in the top left corner. The various accounts you have on your phone will be displayed, along with each of the calendars that are in those accounts. That should give you some clue as to where the other calendars came from. You can hide all calendars and then only turn on one of them to see what events that calendar contains. (List view is most helpful in that regard)
If the extraneous calendars ended up in your iCloud account, you can tap the Edit button, then tap the calendar to bring up the edit screen. Scroll down to the bottom and there will be a Delete Calendar button. Do this only if you're sure that calendar has nothing useful to you. For any events you need to move to another calendar, edit them first. You can pick which calendar they are saved to in the edit screen of the event. I don't know of a way to mass-move them. If the calendar is deleted, any event it contains will also be deleted.
For other kinds of accounts, you would have to turn off "Calendars" in Settings -> Mail, Contacts, Calendars -> account name
