How do I block port 25 in a SonicWall TZ 170 or Exchange 2000?

I am working with a company that has Windows Server 2000 and Exchange Server 2000 and also have a SonicWall TZ 170. They have been blacklisted and are unable to send emails. We were able to get around this by using another service called DYNDNS to route our emails through. The problem is that we are paying per so many messages. The spam is still going out and running up a large tab having to pay per message. I have read other posts and they all mention blocking port 25 for the SMTP. I am unsure of how to block this port. I do now know if I need to block it in the SonicWall or somewhere on the server. If you could please provide details on how to do it either way, whichever is best, please do so. As far as I can tell, there may be an infected machine somewhere on the network but I am not sure where and I am assuming that I need to block the port 25 before it will show in the log which computer is accessing this port. Any help on this matter is greatly appreciated. Thanks in advance.