No, tags should not be use for this.
A spam tag should be something to do with handling/removing/creating/etc spam (eg email) on Ubuntu, and not coerced to a meta-tag for doing something that is infinitely better done through flags.
Edit: It was only created an hour ago, apparently for purpose. I think the justification is far too weak (no, spammers are not going to select the spam tag). I've run it over with my digger.