Fungicides or biocides in any shape or form, dosed too high, can cause normal cellular functions to stop in plants even though the stuff isn't meant for plantlike organisms. This means that whatever fungus you were treating, will probably be dead, but there might occur some weird issues like chlorosis, sudden death, or other nutrient-deficiency like symptoms in your plants too. It might just be the case that nothing happens.
Cleaning up fungicides isn't a pretty job, it's actually pretty hard. That's because they're made to last, for that extra protection.
I don't know the constituents of mancozeb, and the kind/type of product you've used; there are a couple formulations going around. My first mode of attack would be activated charcoal solutions; it can take up a lot of molecules and keep them caught in a matrix of carbon. It will turn the plant pitch black and it will stick in any crevice. Then I'd oxidize the fungicide with something like peroxide or a weak potassium permanganate solution. This is however, next level chemistry stuff that needs a bunch of research first; you want to know if you're not doing more damage than the fungicide itself, and you'd want to make sure that you're not creating a more carcinogenic byproduct than mancozeb already seems to be.
So what's the best thing to do? I'd say wait it out. See what happens. If you have been doing things according to the instructions on the label, then there might not be an issue at all. If others don't stick to the label, then they might be breeding resistant fungi. That's how antibiotics work: you either do it right, or you can throw them away because they've become useless.
