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.

While cell is in edit mode, select text (in the cell or in the type bar), just after you typed it, and then press Enter. This doesn't make any auto correction at all, so you do not have neither to delete anything in your auto correct list nor disable auto correction. Not even have to add space, which may have unwanted results later.