How to turn off "has left the conversation" messages?

We have a small group of people using this excellent product so we didn’t have to use a 3rd party anymore. Most of us are using pidgin or trillian to connect to the server. We’re finding it awkward to adjust socially to the introduction of “has left the conversation” messages which appear when the person you’re chatting with closes their window, so we’d like to disable these messages and allow people to continue in their day blissfully unaware of how their peers choose to manage the windows on their screens.

Is there a simple way to disable the “has left the conversation” messages at the server-level? or if not just a config option, can we filter this particular XMPP packet at the server somehow?

Thanks for your time.