Hi Ian,
To use an icon, you wouldn't you have to select the cell and then
click on the toolbar icon ?
If you are using the mouse, you can use an Event macro to
simply double-click on the cell as show in
http://www.mvps.org/dmcritchie/excel/event.htm#ticks
If just using the keyboard you could use a change event macro
to make anything typed in that is not empty or a space convert to
a capital "P" otherwise have the cell cleared out.
Pre format the column (column 1 used in example) as "wingdings 2"
Private Sub Worksheet_Change(ByVal Target As Range)
'to install -- right-click on the sheettab of the sheet to
' be used in and choose 'view code'. Paste this Worksheet
' event macro into the module.
'-- Column A should be preformatted as "wingdings 2"
If Target.Column <> 1 Then Exit Sub 'check for Column A
If Target.Row = 1 Then Exit Sub
Application.EnableEvents = False
If Trim(Target.Value) <> "" Then
Target.Value = "P"
Else
Target.Formula = "" 'clear out cell
End If
Application.EnableEvents = True
End Sub