Barbarians spawning even though barbarians are turned off

Barbarians might spawn in your territory when your population is very unhappy. You can tell if your population is very unhappy by an angry looking happiness icon. THere are several other consequences from your population being unhappy:

When your happiness is negative and your happiness icon is looking angry, your population is "very unhappy." If your population is very unhappy, your cities stop growing altogether, you cannot build any Settlers, your military units get a nasty combat penalty, and rebel(barbarian) units may spawn inside of your territory.

Source: Civilization V wiki happiness section