When you say "last" I believe you mean new record (the one on the bottom of your continuous or datasheet form (looks like a table but is actually a form). So technically you want to disable or lock the combo-box if the field is populated and enable it if it is null. You can do that in the OnCurrent event of the form or in the OnEnter event of the combo-box itself:
Code:
If IsNull(Me.YourComboName)=True then
Me.YourComboName.Enabled=True
else
Me.YourComboName.Enabled=False
end if
or with one liner like that
Code:
Me.YourComboName.Enabled=isnull(Me.YourComboName)
Cheers,
Vlad