Wow - that's pretty darn annoying.
The best workaround I found is: open a new browser window, and use that to disable JavaScript. At least for IE, Mozilla, and Opera, when you disable JavaScript in one window, all other instances are affected as well.
Unfortunately if I'm using Opera in tabbed mode, I can't seem to get a new window - Ctrl-Alt-N within the existing window is pre-empted by the modal dialog, and if I try to start a new Opera from, say, the desktop icon, it gets translated into a new frame rather than new window - and again, it's pre-empted by the modal dialog.
Mozilla doesn't have this problem though - I can be in tabbed browsing mode, all tabs get locked up by the modal dialogs - but I can still start a separate window using the desktop icon, and disable the eveil JavaScript.
And in IE of course, there's no such thing as tabbed browsing anyway, so of course I can start a new window. It's just much harder to locate the hidden controls to disable the JavaScript. :roll: Still, good to know it's possible. Of course, you can always use the task manager to kill the browser completely - but I often have other tabs with unsaved data, which I don't want to lose.