Say I have a page about strawberries and unicorns. I do want this page to be found by the word strawberry, but I don't want it to be found by the word unicorn - though both words are mentioned frequently on the page.
Is this possible?
No, not really...
If you don't want to be found for the word unicorns then don't publish anything about unicorns (either that or publish the content about unicorns on a private section of your website).
As zigojacko says, this is not really possible. If the word is contained in the page content then you could potentially be found by that word.
However, and this is just a thought, you could encode these words in the page content and run a script when the page loads that decodes them.
However, there are some caveats: