I want to turn off the syntax highlighting for TODO items, like TODO and FIXME. How can I achieve this?
The brute force approach is to nuke it with this:
:hi clear TODO
This may have a side effect you don't expect, though, since it can remove all highlighting from the text in question. For instance, if the TODO text is in a code comment where normal comment text has some other highlighting, then after your run this command the TODO text will appear different (e.g. white on black) than the rest of the comment text (e.g. yellow on black).
Maybe you're okay with that. If you're not someone may come along with a more nuanced solution.
B Layer's solution works. Another option: Since TODOs are often in comments, if you want the TODO to end up having the same highlighting as regular comments, you can do:
:hi link Todo Comment