Turns off mailing list, various types of meetup notifications, etc. Disables it for all the meetups you're signed up for, so be warned! I just put this code into the Javascript console for Chrome. Presumably you can do similar things in Firefox, IE, Opera, et al.

disable_meetups_spam.js

//http://www.meetup.com/account/comm/
$(".D_actions > li > a").each(function(){$(this).click();});
$(":checkbox").each(function(){$(this).prop('checked',false);});
$("div.input > label:last-child").each(function(){$(this).prop("checked",true)});
// I think this is needed for unsubscribing to mailing lists.
$("div.input > label:last-child > input").each(function(){$(this).prop("checked",true)});
//yes, I am a god. Spam-less meetup!

Okay doesn't quite work, there are a lot of save settings buttons.