Those who abuse the system would have their itch accounts banned & an IP ban put in place to deter them from making new ones. Itch already has a system for Two-Factor Authentication so make that a mandatory requirement when making a new account. Even the most ardent trolls will find it cumbersome to go through the entire 'make a new account' process alongside creating new 2FA info & verifying it every single time.
If itch wants to go further to prevent abuse, they could require phone verification with a valid phone number via SMS. That would significantly cut down on a lot of potential abuse. However, that would also exclude a lot of people so I feel the 2FA requirement becoming mandatory would be a nice middle-ground, even for those without phones.