Here's the answer from the FAQ about searching: http://delicious.com/help/faq#searching - "Vegans can use the - (minus) prefix to exclude meat products: lettuce tomato -bacon -ham gets you salads."

In other words, you can search for "tvshow -fanfic" and you should get what you want. That's just a normal keyword search, which takes into account titles, notes, and tags. You can also search for "tvshow -tag:fanfic" if you want to specifically exclude bookmarks with that tag.