Turn off tags for the Google metadata plugin using Configure metadata download. You could even disable that plugin completely if you want. People fall into either the love 'em or hate 'em camp with the tags that it produces, I'm with you in the hate 'em camp.
One approach a lot of people use is to install the
Goodreads metadata download plugin. This plugin is designed to retrieve Goodreads genre shelf names as tags - so if you like your tags being things like "Science Fiction", "Fantasy" etc without all the other rubbish of character names, locations, advertisement plugs and who knows what else then this should give you what you want.
Additionally it has something called "tag mapping" which lets you map Goodreads shelf names to your own desired naming convention, if you don't like what Goodreads calls them. So if you want "YA" to map to "Young Adult", or don't want to receive "Contemporary" you have control over that. The plugin comes with around 60 of the most common Goodreads shelves mapped (from the genres of books I tend to work with), and dwathny has expanded on this to map several hundred tags if you want a more substantive set of mappings to start with. His post with instructions is on the first page of that Goodreads thread (see post#15).