How to get force gate to ground if pwm is low
First off the IRFZ46N is an N-Channel MOSFET. If you want to keep the MOSFET completely OFF you need to have the GATE drive signal of the device LOW at GND and not high at a positive voltage.

This circuit does produce a PWM signal at frequencies of ~220Hz at the extreme ends of the POT and ~180Hz at the mid position of the POT. I simulated with operation at 12V applied to the circuit. This is kind of a low frequency for PWM if you want quite operation. Typically for quite operation a motor PWM is generated at something more like 20KHz being above the audible range for most folks. 

Another potential problem with this circuit is the technique used to drive the gate of the MOSFET. The IRFZ46N has an input gate capacitance of ~1700pF. The DIS (discharge) output of the 555 chip is an open drain pin in that it will only actively pull down toward GND. The turn-on of the MOSFET is only controlled by the R2 10K pullup resistor. At extreme limits of the PWN the MOSFET may not switch very well due to poor waveform at the gate. You may need to reduce the R2 resistor value to make the output waveform more of a square wave.

In Conclusion. 
This circuit will always produce some small pulse at the ends of the POT adjustment range. If you tried to add some additional circuit to force the 555 DIS pin low all the time then it is unlikely that the circuit would restart running again. Instead you probably want to add a separate circuit with a comparator that detected the near 0 PWM setting of the POT that would drive an small NPN transistor that would time into the MOSFET gate and force the gate low until the POT was turned to some setting above the lowest.