One-time-use phone number

You could sign up for a service where you'd get a list of, say, a dozen unlisted phone numbers. You could give them out on product registration forms, to potentially scary blind dates, or other people you may want to avoid in the future.

If the recipient of a particular number proves to be someone you'd like to avoid, you would call the provider and ask that the number be disconnected (giving the standard "no longer in service" message -- as though you had moved or hadn't paid your bill on time).

To avoid cluttering up the public numbering plan, this could also be implemented as a sparsely-populated set of extensions from one central number, like voice mailboxes. Depending on turnover, the numbers could be recycled periodically.