Give her a prison outfit that prevents her from morphing

Reward: lifetime of sadness  
Lol

On the other hand, when you think about it, changelings would make good cops…

Pinkie Pie: Hi, Welcome to Good Burger home of Good Burger can I take your order.
 
Gallus: I liked Two Good Burger, Three Good Fries and Good Shake.
 
Pinkie Pie: Two Good Byrger, Three Good Fries and Good Shake.
 
Meanwhile
 
Ocellus: Now it is time to disguise to Gallus.
 
(Ocellus disguise Gallus)
 
Ocellus: Perfect.
 
Big McIntosh: Howdy Gallus What can I do for you
 
Ocellus: (Gallus voice) Well about only this.
 
(Gallus Punches BIg McIntosh and Gallus Runaway)
 
(Turning Back)
 
Ocellus: Brilliant.
 
Meanwhile
 
Gallus: That was a good food.
 
Police Officer: Freeze.
 
Gallus: I Can Explain, Officer.
 
(Police Officer Hand Cuff to Gallus and Gallus go to Jail)
 
Gallus: I hate it.
 
4 Days Later
 
Police Officer: It’s time to executed, Gallus.
 
Gallus: Uh-No.
 
At Electric Chair
 
Gallus: This is Not Good.
 
(Gallus Sits Electric Chair and Tied Up)
 
Police Officer: Any Last Word Gallus.
 
Gallus: I’m Not Dying.
 
Police Officer: Pull the Switch Up.
 
Police Officer: Wait, Look what I found.
 
Gallus: Ocellus.
 
(Gallus Untied Electric Chair So Ocellus Sits Electric Chair and Tied Up and Pull the Switch Up and Ocellus Died)