If that one cell were (for example) D50, you might try:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$D$50" Then [A1].Select
You will need to incorporate this into the existing worksheet change event if that's what you're using to make all the other changes. If those happen purely through formula calculations, then this is all you should need in the sheet module.
Just another option to play with, you could lock all cells and using the Control Toolbox, select the whole sheet and set the "EnableSelection" option to "1 - xlUnlockedCells" only and then reprotect the sheet, the cursor won't be able to move from there then.