Someone once stole a laundry basket of mine. So I wrote my name and apartment number in permanent marker on the bottom surface of my laundry basket - and caught the culprit the NEXT time it happened. It seems like such an utterly stupid thing to steal... same goes for laundry detergent. What used to get me upset more than anything was morons who would take my wet clothes out of the dryer and then put their clothes in and dry them with my money. I caught someone doing it and boy, were they embarrassed. I took their clothes hostage until they gave me back my money (put them in a laundry basket and sat on it while I waited for my clothes to dry and for them to come back), and then I reported them to the Apartment Manager. It would be interesting to set up a webcam to see exactly who it is that's stealing your laundry supplies.
Keep your laundry supplies in your apartment - top shelf of the front closet is a good place to store it - and only take enough to the laundry room to do your own clothes. Liquid detergent can go in a washed out mayonnaise jar or yogurt tub. Powdered stuff can go in a ziploc bag. Honestly, I really think that some people are dumb enough to assume that laundry supplies left out are "community property". If you don't leave it there, they can't take it. Pretty simple.
It's really worth it to go to the laundromat, put all the clothes in several machines all at once, dry them all at once, fold and sort them, then take them all home. It gives you an hour or more of "personal time" with a good book, a thermos of coffee or tea, plus the peace of mind that nobody is going to steal your stuff.
Oh, and throwing in a red sock in their whites is revenge enough. If it's a guy that's doing it, he'll have some 'splainin' to do about where he got the pink undies, socks and shirts. And the bleach in the detergent bottle is good too. Or maybe pick up some skunk musk in a sporting goods shop, or if you have a cat, put a rag in the litter box to soak it with urine and pop it in the dryer. [Disclaimer: Those are revenge fantasies, and only for entertainment value, not recommended to use IRL.] Having to replace their wardrobe just once should teach them that it's better to pay for their own supplies. How would they go about explaining that you ruined all their clothes, when they were the ones who stole your laundry supplies in the first place?