Unfortunately, OS X does not handle text clippings very well. The problem is actually because Spotlight does not index the contents of text clippings. If they come up in Spotlight searches, it's because of the file name and not the contents. As a test, change the name of the file to something totally different. You'll see that it no longer will show up in searches for words in the clipping.
I actually have been playing with a Spotlight importer for text clippings in my spare time. If I get around to finishing it, I'll probably post it up as a freeware download. I don't have a timetable now for when this will happen but feel free to email me if you are interested in testing it out.