During the account creation process, the user either pays for their account to be created or they use a free service provided by someone else who makes verifications before creating the account. So it's the responsibility of whoever is creating the blockchain account: https://signup.hive.io/
And one person using multiple accounts is perfectly fine. There are other things which are problematic, such as abusing the reward pool, and there are groups that watch for that and intervene when necessary.
Feel free to ask questions, you'll find a lot of help around. :)