When someone's ignoring your very well-meaning efforts to contact them by mail, perhaps tossing your letter immediately into the trash, or maybe allowing it to become buried on their desk, entirely forgotten, while you're watching the mailbox (in case they write) and waiting for the phone to ring (in case they call), and
When you aren't terribly worried about hurting their feelings, and you're getting tired of being ignored and starting to be a little suspicious that something fishy might be going on, because why else would they refuse to contact you in response to your polite and well-intentioned inquiries, and
When you've decided that enough is enough, no more Ms. Nice Guy, and choose to take more direct action instead . . .