Hey guys, just thought I should post this here because I got the PM too.
The member that you're dealing with is not a person sitting at a computer, it's a bot.
A bot is a program that is programmed to do certain tasks - sometimes good, sometimes bad. Most are harmless, like spiders, and are used just to analyze what files and data are on a website.
The one you're dealing with, however, is a malicious bot.
http://en.wikipedia.org/wiki/Internet_bot#Malicious_purposes^ See there for more details
More specifically, the Virus/worm. The bot is programmed to send out that PM - which is why it is always the same and is signed "Forum Member" so it can be sent out to ANY forum.
The best advice I can give you guys, speaking from personal experience, is to ban the IP and email (if you can), and make a general announcement that the entire site can see which would hopefully prevent anyone from clicking on the link.
Outside of that I don't know what else you could do, I'm a mod on a vBulletin forum and have never been staff on a SMF site, so I don't know what your Control Panel looks like/is capable of.
Best of luck to you guys; Jasmine (I'm assuming you're the owner), if the bot persists I'd suggest you contact the Simple Machines Community and if they could possibly help you.