Why does Excel 2007 changes the text "HSA"to "HAS"?
I would remove that auto-correct word from the list like this:
Office Button > Excel Options > Proofing > Autocorrect Options and scroll down in the list until you get to "hsa", highlight it, and click delete.
You can't really disable it at a cell, but you can at least disable it for that one word you are having trouble with. You can always add it back again later.
File -> Options -> Proofing -> "AutoCorrect Options" button -> "AutoCorrect" tab.
It has hsa -> has correction rule. Remove it.
P.S. The path is for Office 2010, but should be the same for 2007.