Do a search for gnats or fruit flies on the forum here. You'll get lots of results and suggestions.

Flip over the material top to bottom in one fell flip. Cover the entire top with a few sheets of newspaper. Let the top dry out. Use sticky traps to capture flies. Stop feeding for a while. Use this material outside and start over, waiting a month to break the cycle.

I found an interesting solution for those tiny flies. We have recently been graced with the visitation of hummingbirds, and whenever I have a lot of fruit flies, I open the bin, and the hummingbirds eat them! Problem solved!

Before the hummingbirds, I would just bury the food under some extra bedding - that seemed to reduce the numbers.