Check out the event lot page in the ACP (under advanced). The user's deletion event should have been logged, and the IP address should have been saved.
... as long as your NodeBB is new enough, that is
is there a way to limit the registration per IP to avoid spam (manually registrations with fake emails)?
If a user has registered one account, he will not be able to register a second one with the same IP (optionally for X time)