How can I lock a cell containing a formula without locking every other cell in the spreadsheet?
Select the resulting range and remove protection from cells other than the formula cell (see: Format Cells… >> Cell Protection: Protected), and protect the sheet (see: Tools >> Protect Sheet…).
@eeigor is perfectly clear here, but it might be worth framing it just a little more. You don’t just protect or not protect individual cells. You effectively format them as “protected” or “unprotected” then you turn on protection for the sheet, and that activates the status of each cell, sort of baking the protection in until you unprotect the sheet again.