A family of Microsoft relational database management systems designed for ease of use.
Probably the simplest way is to conditionally lock/disable the subform control by adding this to the code I gave you earlier:
With Me.[YourSubFormControlNameGoesHere]
.Enabled = Not Me.Sent Or Me.NewRecord
.Locked = Me.Sent And Not Me.NewRecord
End With
The subform control name is the name of the control in the parent form which 'houses' the subform. It might or might not be the same as its source form object.