Dresden uses thaumaturgy in many of the books, usually to generate a tracking spell, though he did use it a few times to bind an enemy (Loup Garou, the Titan).
However, he always describes how dangerous it would be for someone to gain possession of a wizards blood, hair, or fingernail clippings. In the very first book, Marcone's thug chops off a bit of Harry's hair and runs off, and Harry absolutely *panics*. It appears that there is no magical defense possible if someone has a thaumaturgical link to you.
**So why isn't it used as a weapon more often?** We do see characters taking care to clean up their blood and burn bloody rags, but it seems like it should be fairly easy to bleed a wizard or cut their hair and then use that against them. In fact, we see the Redcap do this in Cold Days (though he only uses it to track Harry instead of murder Him because he's a double agent).
This question will be discussed on the next episode of
Recorded Neutral Territory, with the most insightful answers being featured on the show.
RNT is a chapter-by-chapter re-read podcast for the Dresden Files.
Episode 7 (released today) discusses chapters 13, and 14 of Storm Front along with a question: What makes for a great fight scene?
Youtube Link to Episode 6
