I had a margarine theft issue at my last place of work. It sounds petty and I don’t really begrudge someone a bit of marge, but when I buy a full tub and get two uses out of it before coming back an empty tub in the fridge(*), it’s really not on.
I fixed it by lacing a mostly used tub with a metric sh!tload of salt. A couple of days of finding toast and sandwiches in the bin, and no-one stole it again.
(* – and what’s up with that anyway, people are happy to use up all my stuff but can’t throw away the empty box because that’s some sort of bigger crime? Weirdos)