Release a frog! Just kidding. I have noticed that wild and loose flies like to hangout on my ff cultures as well. I just chatch them when I feed if possible, or leave them as they aren't in great numbers. You could fill an open container with smashed up fruits, or medium and they should all go to it. Then cover it up and destroy it or try and culture them. You are going to have to dust them so they can't fly, and that can be tricky lol. Hope this helps somewhat.
you can make a funnel trap with a pice of pineapple (or other fruit... but the pineapple didn't rot for me) sitting in some vinegar/water and one drop of dishsoap (lowers the surface tension so the flies drown).
Put the bait in a small jar, make a paper funnel using tape and tape to the jar. Very effective.
Another thing I have noticed about my loose ff's is that they love to gather around water. If I have something full of water, that is open, they will all gather around the rim of it. Some fall in and drown, but I just knock the rest into my dusting cup during feeding.