Hi,
we ran into similar issues with Zammad: Add User Attribute "Ignore tickets from user" e.g. if user is known for spamming
It looks like the current only way is to set up and e-mail filter and to set users inactive.
Unfortunately inactive users can still create tickets with certain interfaces: https://github.com/zammad/zammad/issues/2357
But from the git thread I take that they are looking into implementing some way of blocking users so all new ticket attempts from all channels would be blocked once you set a user as blocked.
Best regards,
Nino