Excel: Confine Input to Specific Cells Only

Select the cells to which you want users to have access.
If the cells are not adjacent, press and hold the Ctrl key while clicking on each cell.

Right-click the selection and choose Format Cells.

 

Go to the Protection tab. Clear the check box next to Locked and click OK.

This prevents these cells from being locked when you enable protection.

Go to the Review tab

Click Protect Sheet in the Changes group.

Select Protect Worksheet and Contents of Locked Cells.

Under Allow All Users of This Worksheet to, make sure only Select Unlocked Cells is selected.

Click OK.
Users will now be unable to enter anything into any cell besides the ones you selected in the first step.