The service then links the token to your account and uses ordinary detection measures to see if you're spamming, flooding, phishing, whatever. If you do, the token gets blacklisted and you can no longer sign on to that service.
This isn't foolproof - you could still bribe random people on the street to be men/mules in the middle and do your flooding through them - but it's much harder than just spinning up ten thousand bots on a residential proxy.