I pointed out to sitters that I’d bought new jams and condiments rather than leave the rag ends, but they were welcome to use them and other pantry items - even though they were unopened. I returned to find the sitters had taken all the items with them.
You never know what other people are going to think, so I’ve learned to be specific in writing.
Absolutely. I had a sitter just recently who ended up taking my pet camera cord instead of her iPhone cord or something. She has blocked me and won’t return it even when I asked for it. I also noticed she left all sorts of dirty containers in the recycling bin, but like you, I assumed that she would know not to put dirty things in there. I had to sort everything out. I had to show a rare cardboard went before I left, and she put it in that van and then wondered why they didn’t take it. I also noticed my erasable pens missing a whole package. She put dirty towels and rolled them back up and put them in my cabinet didn’t leave me one clean towel. I got her some yogurt, bread, chocolate, oat milk. I got back to zero milk or oat milk in the fridge for me.
I’ve had several sitters take my sheets as well. I think I’m going to do a checklist before they leave next time as I’ve had this happen a few times. I also like you say to help themselves to condiments oil spices, so those kinds of things. It came back to crumbs in the butter. 
A dirty air fryer And no clean sheets on my bed as she had stripped them, and I had to spend the whole day washing.
We had gone over cleanliness, but you would expect that people would at least leave you a clean sheet and a clean towel especially after you return from a long trip, but I’ve learned that I definitely need a checkout list and not to let the Sitter go until I’ve done so. But you do notice things even after they’re gone. Like the pens, I couldn’t find them. They have gone missing. Did I mention she lost my house key??!! When I went to go change the rating on the review, it said that two weeks had gone by already and I couldn’t change it. But the sit had just finished so I’m not sure what happened there. So anyway yes definitely put a review. It sounds like they really took advantage and ate everything without buying any food. That’s not OK. They should’ve asked you before they went ahead and did that as that’s very expensive as you have noted. I’m definitely doing a check out last I think trusted house. There should have one on there for sitters as there doesn’t seem to be at the moment so I will make my own up.
I get that we’re getting them for free, but people need to be responsible. And don’t feel bad for giving a bad review.
With so many sitters using THS , there are bound to be a few bad apples, but for you to get so many makes me think there is something about you, or your listing that is attracting bad sitters.
Sitters taking sheets has to be extremely rare, yet you have had several - it’s almost unbelievable?
Once you have left a review, you can not go back to change your rating.
Could you add a link to your profile, maybe experienced sitters will be able to offer you some advice on how to stop attracting bad sitters.
I agree with Colin. Something is definitely wrong with your THS posting or your choices of sitters. Do you look at sitters’ reviews? Do you spend the time to Zoom or FaceTime with applicants?
What you describe is beyond “bad sitters!”