For the fridge, I liked a former company that had a policy that you label your lunch with a dated sticker. Every friday the fridge gets cleaned out. If you don't have a dated sticker, dated within the last two days, it gets tossed. Doesn't matter if it's in a lunch box or if you were storing bars of gold in the fridge.

If you have a fridge with lighter use, you can do the same think with blue and red stickers. Week 1 is a blue sticker week. Put your food in the fridge with a blue sticker. On Monday, everything with a red sticker gets tossed, blue stickers stay. The next week you switch colors and then switch back the week after that.

Only exception might be condiments which, if not labeled, are assumed to be for communal use.

Edit: By the way, I think the biggest issue, by far, is not people being inconsiderate, but merely forgetting that they had put stuff in the fridge in the first place. They buy a plumb, put in the fridge and then forget about it altogether until it goes rotten.