Rice cooker not stopping?
Most ricer cookers use temperature of the pan - which is regulated by water content! - to decide when to stop. Only when enough water has evaporated to leave you with reasonably dry, palatable grains can the pan temperature exceed 100°C/212°F. If what you are cooking cannot absorb all the water, the device will cook until all the water has evaporated (which the lid makes very slow).

If actually boiling it dry doesn't stop it, and no "keep warm" or "saute" or similar special mode has been selected, return it, it is broken.