Although I imagine most people here know, I'll add this for people that find this thread in the future via search engines:
This isn't just an IGH thing. Every hotel group will treat you the same way. If a hotel discovers they weren't paid in full (or even just believes that to be the case), they will deduct points from your account after the fact. Chances are strong that no one will contact you when that happens.
So, consider that fact if you are contemplating leaving IHG over this type of incident. The grass isn't any greener at a different hotel group.
Additionally, this is something done by individual hotels (that are almost always franchises). As such, the best course of action is to never stay at the hotel that did this without notifying you. But be certain to research the owner. It's not unusual for a local company to own multiple hotels of different flags in the same region. Moving your business to the nearby Marriott/Hilton/etc might mean you're still giving money to the same owner.