Well, I think I got it figured out. This may not be the best
way, but I'll share it with the community anyway in case anyone
else needs it.
1. In RoboHelp go to Tools > Reports > Broken links.
2. Save the report to a text file.
3. Close RoboHelp (it'll crash later if you don't)
4. Find the .hhk file in the project folder (back it up
before starting)
5. Open the .hhk file with WordPad.
6. Copy part of the "Missing Topic" path from the report and
search for it in the .hhk file.
7. When you find the keyword reference, delete everything
between the <li><object
type="text/sitemap"></object> tags.
8. Save the .hhk
Note: after doing this, I had to restart my computer before I
could open a topic in RH without it crashing.