The combobox needs to be unbound from the subform's recordsource, (but bound
to its own rowsource). Use the NotInList event to add new records. Try this
KB article:

Use NotInList Event to Add a Record to Combo Box
http://support.microsoft.com/kb/197526/en-us

try putting the following code in form's Current event procedure, as

   Me!ComboboxName.Locked = Not Me.NewRecord

replace ComboboxName with the correct name of the combobox control, of
course. alternately, you can use the following code in the combobox
control's Enter event procedure, to prevent changes once data has been
entered, as

   Me!ComboboxName.Locked = Not IsNull(Me!ComboboxName)

that's a very harsh environment for a user to work in, though, because it
doesn't allow for a data entry error at all. simply locking the control
after a new record is saved and the user moves on, is much more forgiving of
human error - though you may want to consider a contingency for the user who
doesn't notice a data entry error until after exiting a new record; that
will undoubtedly happen from time to time.

hth