It might seem obvious, but it's a good place to start. Buy yourself a chunky marker pen and start writing your name on everything you put in the fridge and cupboards.
A great way to deter people from snacking on your food is to wrap everything up in a plastic bag before you put it in the fridge. Just by making it less obvious what's inside is sometimes enough to deter people from stealing. If your thief is a lazy person, they aren't going to bother looking inside.
Find some sneaky hiding places around your kitchen to store your favourite treats. You could try using some decoy containers or hiding things in your room instead.
This one's a bit extreme, but you can actually buy lockable food storage containers to put in your fridge.
Pretty self explanatory. This might make them think twice next time they're snooping around your ingredients.
If you've tried everything and your thief is still at large you could always leave out an extra special treat for them, and use some chilli powder or hot sauce to give it an extra kick. It might be enough to put them off next time.