This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
You're not generating an Excel document, you're generating HTML, which some spreadsheet applications (like Excel) happen to be able to read. If you want to use actual spreadsheet-like functionality, you'll need to generate actual XLS or XLSX documents, possibly using a library like Apache POI.
I'm not sure it's possible to protect individual columns, though; I seem to recall that being a per-sheet (or even per-file) setting.