Thank's for the idea.
After thinking a while, I'm not going to implement this feature, because deleting of messages will not be enough. There should be some logging too, and deleted messages should be stored to some trash folder. If some number is blacklisted by accident, or is not whitelisted even it was assumed to be, it's important that messages are still available. Also, in practical cases messages from some numbers are handled differently than messages from other numbers. As you wrote, eventhandler script can do the B&W listing easily, and it's a natural place for handling, because other handling is done in that script too. And it's better if all handling is done in the same place.
In this post:
[answered] How to prevent receiving SMS from certain number there are some samples for handling of numbers.
