Signing up other people for accounts to mess with their inbox is a well-known abuse vector. The issue here is volume, not intent. Other than doing IP reputation, email domain reputation, and rate moni...
(You also need to account for the surprisingly not insignificant percent of people who type their email address incorrectly and -that- email can get delivered to a real person... [and of course you then get a second signup within minutes from the same IP for a very similar account name])