First of all, may I say that it is tremendously gratifying (redeeming?) to hear Carrol call for people to use plain text. I stand vindicated! Second, Dwayne beat me to it with Thunderbird instructions, so I will offer a small addition with a caveat:
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Thunderbird 1.5.x
Go to Preferences (Thunderbird top menu item) in MacOS X or Tools->Options (or is it Preferences?) in Windows.
Go to Display->Fonts and check "Use the default character encoding in replies".
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What this does is avoid Carrol's second problem: once a "user" sends an unnecessarily formatted message (i.e., non-text message) replies to it (depending on mail reader) will propagate his error. Checking this box will prevent that. But at a cost, so please use with care (and perhaps do not use at all if you communicate in languages that require other than Western encoding).
This is a problem that is a bit different from HTML or rich-text formatted messages and gets into issues of encoding, which are unfortunately used in an overlapping manner with text formatting. Thus necessitating the use of force for encoding in responses. If all this sounds unclear technical crap, ignore it.
Next: Jordan can probably drop or return non-text formatted messages even before they hit Mailman (in case Mailman does not support such filtering -- which I sort of 99% know it does) using procmail or equivalent filters for incoming mail.
Final: nobody has posted instructions for Opera (AFAIK). I will try to do so when I have better access to the net (and hence Opera download) than I do right now (14Kbps).
--ravi