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.
For example:
<span class="encoded">snrocinu</span>
However, there are some caveats:
It's less accessible. Some users (JS disabled or whatever) may just see the encoded word.
Search engines are getting better at interpreting JavaScript, so your "encoded" words may not be encoded after all.
If you have more than a few words to encode, this is going to be time consuming to implement (ie. write content). Unless you automate this process as well.