Couldn't you limit it based on credit card number, a corresponding phone number, a Facebook ID, verified address, etc.? Even if it doesn't fully limit it, it perhaps makes it harder.
No. That would defeat the entire user model, which is purpsoefully derived from Bitcoin's anonymity structure.
To be pedantic, BitCoin (and most other cryptocoins) are pseudonymous, not anonymous. But yes, one user can have multiple pseudonyms.
That would require trusted centralized verification.