I tried to search "American Football" -soccer , but there are still images of soccer in the list.
I know its a problem with translations, but there must be a solution for it.
Can anyone help me finding the correct search pattern?
Your search input looks proper "American Football" -soccer, and I wasn't able to see any soccer, you can also try football -soccer. The reason why you may saw soccer related content (though I wasn't able to see any), is that the asset has keyword for "american football" & "soccer" as our search works based on keywords.