You will need VBA to do this and you will need to modify this code to suit your situation.
Right click on your sheet tab, left click on View Code, and paste this in:
Private Sub Worksheet_Calculate()
If [A1] = 5 Then
[A1] = [A1]
End If
End Sub
This code demonstrates the example that, let's say, you have a formula in A1 such as
=B1+C1
The first time that a combination of numbers is entered into B1 and C1 that equals 5, then a 5 will be displayed in A1 and the formula in A1 will be overridden.
Hopefully this will help point you in the direction to accomplish this for your specific cell and True False equation.
Tom Urtis