If you see a number of spam tickets coming into the
helpdesk app from an email account, you can block that email-address. Open the user's profile, click "edit", check "disabled" and click "save". The account becomes disabled and all emails from it are ignored now.
You can also simply click the "Mark as spam" link on the ticket page, which essentially does the same thing. It will delete the ticket and disable this email account so no future tickets are created.
Blocking email from particular domains
You can optionally disable incoming emails from specific domains. Just head over to "Administrator - Email Settings - Accept emails from unregistered users" and click "manage exceptions".
From there you can block all incoming emails from particular domains, or just white-list only the domains you work with.
Hosted version notes
The "hosted" version of the ticketing system has a built-in email filter on the default mailbox that we provide for you (the one that goes "support@XXX.jitbit.com"). Our mail servers check senders IP-address against blacklists, block unknown servers, broken SPF records (anti-spoofing) and run a ton of other check to prevent you from being flooded by spam tickets.