A host in my network was infected and sent spam. How do I stop this in the future so I don't get blacklisted?
This is never a greats scenario, but there are ways to prevent infected hosts from getting you blacklisted. The first step in this process is to create an SMTP policy that only allows your mail server(s) to send SMTP traffic out to the internet and the create a second policy (below the first one) to deny all other hosts from sending out SMTP traffic. The Knowledge Base article below covers the necessary steps to ensure only your mail server(s) are sending out emails as well as some other recommendations:
Eugene Torre | Support Engineer
Apply spamBlocker actions to outgoing SMTP traffic to stop spam from leaving your network. Ensure incoming web and SMTP traffic is filtered by WatchGuard Subscription Services.
See the following kbase article: https://watchguardsupport.secure.force.com/publicKB?type=KBArticle&SFDCID=kA2F0000000QBnRKAW&lang=en_US