In a real board the 555 is triggered to turn on when the voltage at pin 2 is below 1/3 Vcc, if it goes above that it will not trigger and it will urn off after the delay period.
Your transistor in your circuit can only control the relay and the relay is wired to invert the motor polarity, not stop the motor.
If you want to stop the motor then you should remove the power supply from one position of the relay so that it can stop the motor.
You can either use the relay to control the motor or the transistor directly, with the relay you will have lower losses.
If you are going to use the transistor then depending on the motor current you will need more current to the base of the transistor.
A third solution is a mosfet, which will give the lower losses possible with no moving parts (relay)
One more note, you need to use a resistor in the base of the transistor to limit the base current
Alex