Add to worksheet module:

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$B$1" And Target.Value = 1 Then Range("A1") =
""
'if A1 is changed and already B1=1, then A1 won't be deleted.
'you may want to enable the following line.
'If Target.Address = "$A$1" Then Range("B1") = ""
End Sub

Hth,
Merjet