Using Excel 2007 on Windows 7

I know how to change a formula to a static figure by pressing F9. This is
great for a single cell but is there something that could be used in a range.

For example:

Single Cell - =sum(A1:A3)
Cell shows 51
Select the Cell in the formula bar and press F9 changes the =sum(A1:A3) to 51.

What I would Like to do for a given Month is be able to change formulas to
static figures:

For Example:
for each month there are three calculations. once the month is completed I
would like to stop the month from recalculating as the input cells may change
but only for the future months.

Has anyone come across this before?