It's best to not put them there, but they consist of oil a little salt, some organic material (skin cells) and whatever is on your fingers Several things dissolve oil which is primarily the th pattern of your print. Isopropyl or any high strength alcohol on a napkin and thoroughly wiped would do it on the plastic. Paper is a little trickier because the oils and salt have absorbed into the paper. Your best bet in my opinion is to obscure it and erase it. Dab a napkin in salt water and rub the area. Let it dry and then do the alcohol. If you put the items under a lid and squirt some super glue on a disk also under the lid, after a few hours the CA fumes should reveal any fingerprints. This is how you can test and experiment, or at least be sure they are obscured. If you see them scrape them with a hobby knife until they are useless. The police won't be able to recover them using the method you already used. This is my opinion and doesn't constitute legal advice ( I would only be liable if you paid me for the advice anyway)