You can try reprogramming your key, I'm not sure if this will prevent his key to work though.
1. Turn the car to position 2 (middle) and turn it back to OFF. Do this 5 times without any time gap. So it's going to be OFF, Position 2, OFF, position 2, etc. (5 times total)
2. Remove the key and do the following immediately within 30 seconds.
3. Press and hold UNLOCK and tap the center BMW button 3 times. The doors will unlock and lock to confirm successful programming.
4. Immediately within afterwards, repeat step 3 for your second or third key.