Another idea (more like suggestion),
- Make spoiler tags not quote-able.
I won't go into anything too technical with this one, since I'm not sure what you are planning. But anyway, there are two ways I can think of. One way would be to somehow use the quote tag (since quotes don't quote themselves). Amusingly this works in practice with what we have, like so:Regrettably this will require a plugin to get it to be convenient.
So plan B is, given we have a spoiler tag similar to what we have now, create a plugin, make it hook up to the pre-send filtering (the appropriate word escapes me at the moment =P) and strip away any spoilers in quotes. This will work better because it will eliminate intentional nesting of spoiler tags in quotes not just preventing them from being quoted naturally. However, its probably best to create a similar plugin to strip them right before they pass into posts as well as after things like a preview; so behavior is not just hacked into one spot.
Of course people could still do something like "X wrote:" and write the spoiler outside the quote tag but only idiots would do that (ignorant of the hints).