The cell with the formula can be locked. But that means that when the worksheet
is protected, the user can't change the formula--or overwrite the cell with
something else.
But this doesn't mean that the cell's value will always be the same. If you
change the values in the cells that this formula uses, then the formula will
reevaluate and display something else.
If you want to "lock-in" that value in any cell, you'll have to convert that
cell to a value--and probably lock the cell and protect the worksheet (to stop
prying fingers from easily changing the value).
ps. Be aware that worksheet protection is easily broken. So you can do all
this stuff, but anyone who really wants to will be able unprotect the sheet to
change the value.
