What if there was a required payment of $1 to create an account, the payment having to be made from a credit/debit card in the name of the person creating the account?
As it is hard to gets an unlimited number of credit cards, that should slow down the creating of multiple account.
The money raised could be donated to a charity, with the user being able to choose the charity from a pre-approved list.
(Requiring a mobile phone number will exclude some disabled people, so must not be the only way to create an account.)