How can I tell OS X to flag a name as being misspelled?
Removing words from the built-in dictionary is apparently more difficult than I thought it would be. Since your question is about how to prevent a particular misspelling of your co-worker's name, I have an alternate solution.

You can add "Jennet" to Mac OS X's text substitution list in System Preferences -&gt; Language &amp; Text -&gt; Text



It will automatically correct the spelling for you each time you misspell it. You could add other variant misspellings for completeness, like "Jenet" or even make it auto-capitalize "jennett", but since most other misspelling variants don't have obscure English homonyms these will show up as misspellings.