Should be pretty easy. For most Linksys routers it'll be something like this: Go to the admin interface of your router, (just point your web browser at the router. I think it defaults to 192.168.1.1, but you should be able to tell from your pc by using "tracert google.com": the router should be the first entry) and click on the tab that says, "Access Restrictions" and under that you should see a couple of tabs that say "Blocked Services" with a button underneath that says "Add/Edit Service"
Click the Add/Edit button, and put in the range of ports you want to block. Limewire defaults to 6346.
Unfortunately the off-the-shelf versions don't allow fine grained control. If you post your router hardware model, I'll check for updated firmwares. If you can find something that offers full IPTables support, you can do port whitelisting, which is the best way to go...Block EVERYTHING except the stuff you want.
@Nik: Yep, you're right. But this is about all you can do with an off-the-shelf linksys. Setting up bandwidth throttling and such requires configuring an actual proxy, and that pretty much requires a server, and a whole boat load of knowledge (or a bunch of ready cash).
If I was going to do it, I'd set up a squid proxy to throttle by content, and if that didn't work, I'd just throttle the hell out of my problem users (or fire them).