Indeed, in the UK there is a central registry of blacklisted IMEIs. All the carriers in the UK have signed on to this registry, and when one carrier adds an IMEI to the blacklist it filters down to all the other carriers automatically.
If any carrier who's signed on to the blacklist detects a device with a blacklisted IMEI, it will deny that device access to their network, regardless of what customer is using the device, or what SIM is inserted in the device.
The purpose of the blacklist is to report stolen hardware.
In principle, the program is intended to be preventative: If everybody reports their stolen hardware, then all stolen hardware will be blacklisted, thereby rendering them useless to whoever steals the device, thus minimizing the potential payoff to the potential criminal, so they'll be less likely to commit the crime in the first place.
In practise, it is often possible for criminals to bypass the effects of the blacklist by either hacking the stolen phone to modify the IMEI, or else by exporting the stolen phone to a different market where they don't enforce the blacklist.
The IMEI blacklist was only ever intended to be used when hardware theft is an issue. It was never intended to be used by the carrier as a punitive measure against a customer (unless that customer had actually stolen equipment that still belongs to the carrier, and even then, only to blacklist the specific stolen equipment in question). But an unscrupulous carrier could hypothetically abuse the system to apply it in ways that were not originally intended.
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