It may seem obvious, but add an registration through oauth providers like google and facebook. It should trim down the amount without a lot of impact I guess. If all that abuse is in Russian you might just try to deny service if Russian language is used or add an extra hurdle like browser fingerprinting and rejecting the combo language + fingerprint within a timeframe. Depending on your user demographic you could als…
This. Require auth, it also allows you to add stuff like rate limiting per account to further prevent abuse.