The Russians and the Chinese used to block out the registration after an 'incident' until recently. We reckoned that all airfields kept a paint roller on a very long handle in the shed with the fire trucks.
I seem to recall that the CAAC Trident that plopped into the harbour at Kai Tak in the early 90s had the reg painted out in a very short time, before being covered with a big tarp. My memory though...