CNC-ROUTER TOO HEAVY FOR Z-AXIS?

I RECENTLY UPDATED MY OX-CNC ROUTER TO HOLD A BOSCH ROUTER. EVERYTHING WORKS AS INTENDED, HOWEVER RIGHT BEFORE I HIT PLAY ON MY GCODE, THE Z-AXIS SPINDLE MOVES DOWN, DUE TO THE WEIGHT OF THE ROUTER (GRAVITY) MOVING DOWN THE THREADED ROD. IM TRYING TO FIGURE OUT A WAY TO LOCK THE Z-AXIS ON THE START, SO WHEN I HIT PLAY, THE INITAL FIRST MOVE DOESNT CARVE INTO MY TOP SURFACE. ONCE THE FIRST INITAL CUT IS MADE, EVERYTHING IS FINE, BUT I WILL JUST HAVE A "SCRATCH" FROM THE INITAL MOVE TO THE STARTING POINT OF THE PEICE.