In reply to Stash:
When one of my brothers was at music college, somebody kept stealing a person's bread, so he pee'd in the bread bag and the stealing stopped.
If I was your friend i'd maybe not keep the sandwich in the fridge and leave a note saying why, so the food stealer would know why and other people would too.
Create an atmosphere of suspiscion for the food stealer...(
)
Cheers
Tim