I understand how difficult you dsc's mother can be first hand, but I don't really understand the hair band issue. What harm could have come with him handing them to her or packing them in your dss's bag for her? We have had so many difficult issues with my dh's ex, but you just have to let the little things go, as hard as it is. DH and I will NOT let important things that will direcly concern our time with his ds or his up bringing slide and will call her on anything that might conflict with what is directly best for my sweet dss. However, if there is anything we can do to foster positive, good and easy exchange than we do it. So what, if she wants your dh's attention. He's not really giving her the kind she wants, right? So then, do your best to foster openness and friendliness. This can be so HARD. I know but it is important for the kids involved to see you supporting their mother in whatever way you can. It might seem stupid to you, but you don't want them to think that you are trying to minimize the importance of their mother to them. While she might be the worse mother in the world, she is still their mother. Honestly, I think kids demonstrate more unconditional love than parents. They are going to love her regardless of what she does, so if you look like you are trying to foster good will on all parties, they will really appreciate it.
Actually, if YOU but the disgusting hairbands in a plastic bag, and do an extra measure of kindness every time something like this comes up, it also become effective for getting her to stop it, because she'll notice a woman's touch in the exchange and realize that the extra attention from dh also comes with a little unseen extra attention from you, and she will probably not want that.
That's the best part of this approach. It accomplishes the distancing, while supporting the kids' need to "help" their mother.
I also know that it is imortant for you to have an open dialouge with dh about all of this stuff. Just be EXTRA careful the kids can't overhear it, then let it all out. It is very theraputic. Also before starting a new approach with your dh's ex, talk to him about it. He'll really appreciate the thoughtfulness regarding his kids' needs. Plus, then you can have a consistent plan with her.
Good luck!! It is TOUGH!