Forgetting for a second that it was BM, they have stolen your food for a start. Regardless if it was mistake/prank.

In addition, they have stolen a bodily fluid, are they aware of that? BM is treated like a blood ***** in hospital setting, it's not something to joke around with. It's like sharing a syringe, so if it was a prank. I would put on the fridge "sharing breastmilk is like sharing a syringe, it is a bodily fluid with live organisms - DON'T STEAL MY MILK"

Lastly, I feel soooo bad for you 200ml is a FANTASTIC amount and I am so angry on your behalf and your poor bubs!!!