On fly (I haven't Access available at moment to check the syntax):
To table the form is based on, add a field IsLocked (boolean);
On form you'll have a control booIsLocked. Set the default value for control FALSE, and hide the control (Visible = FALSE);
In AfterUpdate event of combo box add a code row (depending on datatype of combo's source field)
Code:
Me.booIsLocked = (Nz(Me.cbbYourCombo,0)=0 )
or
Code:
Me.booIsLocked = (Nz(Me.cbbYourCombo,"")="" )
;
As next row of AfterUpdate event of combo box and into OnCurrent event of form add code
Depending on you wanting to lock the whole entry, or only the combobox:
Code:
Me.AllowEdits = NOT(Me.booIsLocked)
or
Code:
Me.cbbYourCombo.Enabled = NOT(Me.booIsLocked)
Edit. You can name the table field IsEnabled instead, with according control booIsEnabled, and in formula calculating booIsEnabled, replace "=" with "<>". Then you can drop NOT() function in code setting AllowEdits or Enabled properties.