Well in the end racing is a dangerous job. The drives know this when they get in the cars. There will be rubbing and bumping for position racing woudln't be nearly as entertaining without it.
I think the it would only be excessive and warrant a huge penalty/ban if you intentionally put someone into a wall or out of the race. There is a big difference between a bump to get someone loose coming out of a corner and intentionally running someone off the track. That is something that is obvious to spot as well so there really is no need to ban any form or bumping or rubbing.
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