In https://github.com/keras-team/keras/blob/master/keras/constraints.py 13 we have:
class NonNeg(Constraint):
    """Constrains the weights to be non-negative.
    """

    def __call__(self, w):
        w *= K.cast(K.greater_equal(w, 0.), K.floatx())
        return w

I guess this is just clamping it to minimum of 0.