I need a simple way for employees to label their lunches as we have a problem with people not labeling them and when getting upset when their lunch is thrown out.
I've seen this issue before with communal refrigerators shared by 100 people. Some suggestions that worked well:
Keep several marker pens attached to the refrigerator.
Make the cleanout schedule a regular thing: every other Friday at 3pm, for example.
Post the cleanout schedule on large signs throughout the kitchen.
Post the cleanout schedule in a shared online calendar.
Send several all-hands notifications (email or chat): 2 days before, the day before, that morning, 30 minutes before the cleanout starts.
Also, if there are shared condiments (ketchup, mustard, mayonnaise, pickle slices, etc), label them when they are purchased, and replace them after 6 months or 1 year.
I think the first step is to make it easy for people. The second step is to declare the consequences for not following the rules. Carrot and stick.
If you say, "everyone should label their lunch", someone can reply, "oh, I don't have a marker". So tie a marker to a string and hang it from the refrigerator door. Maybe provide a convenient pad of labels.
You'll still get people who say, "I was just too busy". So fine. You say, "any unlabeled food will be thrown out on such-and-such a schedule". If someone complains that his food was thrown out, you say, "We announced the policy, you didn't label your food, I'm sorry. We can't have molding food left in the refrigerator for months."
Unless this is a huge problem -- your company refrigerator is full to the brim with molding food -- I wouldn't be too hard-nosed about it. Give some leeway.
Oh, the ability to use a stick assumes that you have the authority to impose rules. If you're not the boss or otherwise in charge of the break room, you probably can't just decide to make yourself the Lord High Inquisitor of the break room, make up rules, and start throwing away other people's lunches because they didn't follow your rules. If that's the case, you have to rely on persuasion.
If you're a company that follows a typical work week (M-F), you need to get several clips in 5 different colors, one for each day of the week. Something like a cloths pin could work.
When you put a meal in the frig on a give day, use that day's clip. Anything in there without a clip or the color of a clip you deem to be too old (3 days?) it gets thrown out.
If you get those clips with magnets on them, you can just stick them on the frig for their day.