What operating system?
PDFedit can do this. It's just for linux, I think, however.
Probably there are ways of doing it with ghostscript, but it may take some research.
If they're fairly simple PDFs that will survive conversion, could try converting them with calibre or pdfreflow (--search for these here at Mobileread--) to html or rtf and then modifying them in your favorite WordProcessor (or directly).
It might also be possible to do this with a virtual PDF printer that changed things to black and white.