I can't think of any posts I've seen that have a lot of irrelevant tags, but I could easily have seen some of them before they were tagged. Are there any examples you'd like to point out? (Though I do think JP's answer outlines the best response to these tags.)
Tags are a really useful feature, but I think we could use some clarity about how tags ought to be used. For example, I've seen some posts with lots of irrelevant tags, and I think that tag spam detracts from the usefulness of the tagging system. What norms about tagging posts should we have?
An important note about this is that the tag relevance feature is a special feature designed to counter the exact effect you describe. The way most tagging systems work, if you tag a very popular post with tag "Foo" then it will be shown first to people looking for Foo-related posts. But with our tagging system, users can say "this isn't very relevant" and it will stay with a low "tag relevance". See the Forecasting tag, and notice how the Forecasting newsletter comes above the LTF Fund grant post, despite being less popular overall.
So I'd say to be liberal with your application of tags and let the relevance system do the filtering.