See if this does what you want. Right click on your sheet tab, left click on View Code, and paste this into the module.
Modify for range of interest. Also, before you insert this code, unlock the cells in your range (in this case A1:B10), so they can accept their first and only entry.
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Range("A1:B10")) Is Nothing Or Target.Locked = True Then Exit Sub
Target.Locked = True