With forms (activate Event) you can use something like the following;
the key being the .locked property of the combobox.


-----------------------------------
If Cell = yes then
Me.ComboboxName.locked = true
Else
Me.ComboboxName.locked = False
End if
------------------------------------


If the form needed to change  dynamically you would have to use a
worksheet change event to trap the Cell change and then run the above
again.