I would always be able to prevail (after a while) by asking the singer if she was aware she was scooping. If she was not aware, then I made her be aware.

After regular reminders that the singer gets to choose the embellishments appropriate for the music (enter 26 Italian Songs) over time and maturity and reminders

a repetition and proper modelling by you and other singers, the singer can overcome the habit and not vice versa.

S

 Mindy:

 

I suggest having your singers begin every pitch  on "ti" (tee) .  It is much more difficult to scoop while using this technique.

 

Good luck on eliminating the "scoop"  in choral singing.