You can lock or unlock each cell via:
format|cells|protection tab

But the "lockedness" of the cell doesn't mean much until you protect the sheet.
Tools|protection|protect sheet

Be aware that worksheet protection is easily bypassed and there are lots of
things that aren't available on protected sheets.

locking cells in excel wrote:

how do i protect cells in excel to prevent users from deleting formulae in my
worksheets

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Dave Peterson