Surely carrier A wouldn't let carrier B be able to block an imei belonging to their customers. If carrier B did block it though, you, customer of carrier A, wouldn't be able to roam in the network of carrier B - but still work fine in your home network of carrier A.

However, that results in carriers needing to assert ownership over an imei, which would be come quite a mess - so it very well could not be done.

I do believe national blacklisting registers of imeis, as already done for many years in certain european countries is much better than not doing it though.

That makes sense - an earlier response to my comment about how apple handles things with user PINs is probably a better approach.