Packet Sniffer

Hi,
I have problem on my work network. We have MikroTik ruter with WebFig 5.23 and we make a bridge connection.
One (or more) computer on our network is sending bad traffic outside (on specific site with specific IP address).

How can I reveal that computer (IP)?

Use torch http://wiki.mikrotik.com/wiki/Manual:Troubleshooting_tools#Torch_.28.2Ftool_torch.29 to monitor traffic
or add some firewall rules that match and drop/log those computers http://wiki.mikrotik.com/wiki/Manual:IP/Firewall/Filter and lots of examples in WIKI