Increment Cell On Selection And Lock/Protect From Re-Selection

I tried very hard to design a leave roster for user to mark their leave application. The criteria are as follows:

1. 4 applications per date
2. a region of worksheet (i.e. "A2 to H20) will be defined for users to "click" on the cells (within the defined region) to mark their application.
3. once a cell is clicked (i.e. marked), it cannot be altered.
4. each click will automatically increase the total by 1.

I got the following code to deal with the situation but user can still click on any cell outside the region (in fact I fail to define the region).

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim m
ad = Mid(ActiveCell.Address, 2, 1)
m = Range(ad & 24).Value 'here a formula "CountA(A2,A20)" will be place in the cell (24, c).......................