Okay, I remember reading a thread about the complex form Cleaner.

I did a search and couldn't find it.

Is there an official ruling? Can cleaner reduce over watch to nothing?

I don't see why it can't reduce OS to zero. That doesn't mean it's there forever. I'd say that you still get a 2D6 bump in fifteen minutes.

Agreed. Your Overwatch Score doesn't stop automatically accruing until you reboot.

You could, however, use the Static Veil complex form to stop it from accruing over time.

The Complex Form "Cleaner" is in the Main Book. First Complex form listed in the Resonance Library. Page 252
Yes, it can reduce your Overwatch score to zero.
However, your Overwatch score still increments when you do illegal actions or over time.

There is another Complex Form that eliminates increases over time - Static Veil; Same Page Number I Believe.
Nothing stops increments for illegal actions.