Can i Flag a Tag? What if there is an unnecessary tag created for advertising some product?
I'd just add to Al's answer that if the tag is inappropriate but not spam or offensive — e.g., something off-topic like marketing — it may be better to make a post here on Meta and see what people think. Marketing might be obviously off-topic, but often it's less clear and there might be several alternative actions to discuss. See the questions here tagged tags to get a better idea of this.
Just flag a post that it's on and use the "other" option to explain what's going on.