>There is, it's the downvote button. The distinction of why a downvote is useless.

How about a tag system associated with each posting? Then you can have just a scalar "weight" and a set of keywords (string labels), each with a designated meaning. You press a "plus" button or something, and a menu with optional tagging pops up (tag clouds can be used).

Some tags could be "spam", others could be "didn't read" and users can build their own threshold filters for tags. (0) spam; (4) didn't read; (3) incomplete; ...