Is it possible to lock down (make read-only formula cells?

I want to set up a spreadsheet so that formulas which are put into cells aren't deleted if you delete the content of the cell.

Is it possible to erase the resulting numbers from formulas without erasing the formulas which derive them?

For instance, if cell A10 sums up cells A1 - A9, I'd like to be able to erase cell A10 so that the summed number is deleted but not the sum formula (so it can be re-used without having to be recreated). I realize I could delete the numbers in cells A1 - A9 to get this result, but I'd like to be able to manipulate a spreadsheet quicker than that.

Is this possible?

Just one more related question. It's possible to make cells protected. What does that do?

Thank you.