Blocking Messenger access

Greetings.

I'm very new to Macs and i have problem in my hands. I work in a company for a couple a weeks and we just have Macs in here (well, just on PC). We have a big problem that affects our company and that is the Messenger access by some employees (it's "forbidden" but some people insists to use it). So we want to block that access.

I've made a Google search about blocking Messenger in Mac OS but i haven't found anything conclusive. That's why i'm asking here, if someone knows about a solution. Our network is very simple. We have a router that connects to the internet (adsl) and the computers are all connected thru switches. The router is very basic and doesn't allow me to block the ports Messenger uses. We don't have a server to connect to the internet.

So, does anyone knows a solution to this? A software that we can install in each Mac to block the Messenger access (and also webmessenger from a browser)? Any other ideas? I've heard about BrickHouse but i didn't had the oportunity to try it yet... Do you think it's ideal for this situation? Other alternatives/solutions?

Thanks in advance,
João