That's cool, I wonder if that is the root of the word hinder in English?
I like forhindre for this reason, too. My dictionary says the English word hinder is of Germanic origin and traces it through middle and old English. I would guess that hinder and forhindre have a common, older root. I often feel like Danish sounds a bit like Shakespearean English.
If you want to put the "på" there you could, but the sentence makes perfect sense without it.
Is there a way of preventing it -- sounds more natural in English but it's not acceptable on duolingo