A simple naive bayesian text classifier (e.g. A Plan for Spam) run client-side as a userscript should be more than capable of doing what you want. It would be

A simple naive bayesian text classifier (e.g. A Plan for Spam) run client-side as a userscript should be more than capable of doing what you want. It would be nice if websites had this sort of client-side filtering feature built in, fully controlled by the user, for transparency reasons.