Is it possible to digitally stop a moving motorbike using an Arduino module?
Assuming that you simply want to stop the engine, Gerben's comment has the answer: Put a relay in series with the ignition. Control the relay with the Arduino. 

Opening the relay breaks the circuit. Ignition is now off, and the engine will no longer run.

Of course, the bike can still roll without engine power. If you want to completely immobilize the bike, you need to engage the brakes, or lock the wheels in some other way. And that takes a whole lot more hardware.