You could use a Zener diode of, for instance 3V plugged to a negative voltage rail of -2.7V.

You could increase the gain of your summing amplifier then use a diode, then place a voltage divider.

You could also use a mosfet with a feedback from the output voltage that drives or not the fet.

You could use a transistor as an amplifier clipped. 

Working with signals in that order is tricky. All the components will bring you significant voltage tolerance. I would definitely try to amplify it before conditioning it.