How do I prevent outgoing traffic?

Hi
open System preferences go-to Security then click on firewall. You will see three radio buttons, on my one i have the third selected (set access for specific services), this controls the programs that transmit to the internet by giving you control. Underneath this you will see a list of programs if the messenger is not there then you can add apps by clicking the + button. Make sure messenger is selected and on the right you will see some text you can click on the double arrows and select Block or allow connections.

The firewall is for incoming connections.
I know [Little Snitch|http://www.obdev.at/products/littlesnitch/index.html] will monitor outgoing connections, but I am unsure whether you can block those connections, but I think so.

I know Little Snitch will monitor outgoing connections, but I am unsure whether you can block those connections, but I think so.

Little snitch is a firewall that can block outgoing connections. It is application aware. So you can block a specific App on port 80 but allow other apps out on port 80 for example.