You'd have to part before disconnecting. If you don't want to manually do that, then you can make an alias that will do it.

Code:
alias disconnect { part #whatever_channel | quit }


That, of course, won't cause you to part the channel if you're disconnected for some other reason, such as netsplit, ping time out, etc. on DISCONNECT only triggers WHEN you disconnect and by then it's too late to part a channel.

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