Although they don't have a sting, meliponines protect their nests very efficiently. The most common mechanism of defense among stingless bees is the habit of getting through the hair and fur of the invaders, biting their skin with the mandibles, applying resin, and trying to enter the nose and ears. Some species only glue resin on the intruder and others fly around and occasionally bite with their mandibles.
Many species defend themselves by entering the nest when attacked.
In this case, the difficulty of entering the nest is the the main defense against invasion. Other forms, perhaps less effectively protected, nest in or beside nests of ants or termites, from which they presumably gain protection. Other species build their nests next to the nests of aggressive bees or wasps, using their neighbors as a defense.