I feel the best way to deal with this type of crap is to pretend like they're some unimportant random person.

Hello. A request has been created. A member of our staff will respond to your enquiry within 10 working days.

Do not respond at all. When they send another email, auto-reply with the same message. Then reply this after their third email:

We have detected an unusually large number of enquiries sent from your IP address. To prevent spam, further emails will be filtered. If this is in error, please write to P.O. Box 12345, Somewhere, Some Country.

The P.O. box number given doesn't exist. But international mail is slow so it will take them two months to realise that.

But blocking the instance at the DNS level does not stop the content from reaching other Russian instances, right? They would have to basically track every server that is federating with them and block like this.