2 guys at work messed up pretty bad last Saturday.. They are still being punished

I work at a pet food plant, and last weekend the 2 people that receive ingredient trucks on A (One of 2 morning shifts, we work 12 hour days.) We have 2 types of fat, one with a Synthetic Preservative, and one with a natural preservative. A tank for each, that can each hold approx. a load and a half. These guys unloaded the Natural preservative Fat (Way more expensive) into the wrong tank, and to make it worse, they didn't watch it, which is a HUGE no no, and it ended up overflowing outside. Somewhere around 6000 pounds of fat ended up on the ground. Plant management made the call hold them accountable and have them be the only ones that clean it up. They then proceeded to procrastinate, until they got 2 days off. The fat sat there. They came in yesterday and were told to get it cleaned up, or they will be written up for a second time. They did nothing. I took over as a supervisor today and of course I had to basically threaten them to go start it. This fed up the plant manager, and he told them that it had to be done today, or they will lose their jobs, he didn't care how long it took, they would stay and get it done or be gone, since they were being lazy. I offered to help them to get it done in a timely manner, but the Plant manager told me not to, as it was their fault they were in this situation. It has now been 15 1/2 hours, and I just got a call that they are still there. I hope they learn their lesson.