There is currently no way to do exactly what you are asking.However, there are a couple things that can be done to prevent spamming. The easiest is to set a low value for Admin panel -> Tickets -> Maximum Open Tickets. You can enable CAPTCHA's at Admin panel -> Tickets -> Human Verification.Outside of osTicket you could setup a generic user authentication by configuring your webserver to help protect the web interface. How to do this varies by webserver software. You could setup a spam filter/firewall for your email server (Such as the Barracuda Spam Firewall) to rate limit emails, amongst other antispam features that are built in.
