I doubt this is a significant concern for most boards.
A packet-sniffer can't just arbitrarily sniff packets. It can only sniff packets that the hacker has access to. The hacker has to be "on the wire" somewhere between your website and your user. There are surprisingly few such access points - especially since the wholesale switch from Ethernet hubs to switches many years ago.
Maybe 10-15 years ago, (when hubs were still used) somebody at a company could sniff packets from adjoining cubicles (on the same hub), or somebody in a dorm room could sniff packets from adjoining dorm rooms. Or an insider at your ISP, etc.
The only significant sniffing threat today would come from an insider at an ISP or transport provider (ATT, Level3, etc.) who would have administrative access to switches, and access to a physical switch port. (Most switches can be set to echo all traffic to a given port for diagnostic purposes. This allows "sniffing" on switches.)
Unless your board is hosting discussions by the Federal Reserve or the Group of Seven, I doubt you have to worry much about this. :)
Your user accounts are much more likely to be hacked by simpler exploits - spyware installed on their computers or on public Internet terminals.
Has nothing to do with your selection of message board software. You do this with robots.txt, firewalling, web server plugins, or all of the above. robots.txt can be set-up to make "good spiders" go away. "bad spiders" you can ban individually, if you know their IP addresses (updated lists are available), and there are firewalling solutions and web server plugins to detect spider-like activity and ban others on the fly.