chemical to kill flies on contact

C EM DIE, appears to be discontinued, only found an empty vintage can for sale, lol

I second the dawn dishwashing liquid recipe, That stuff can work miracles for numerous uses.

I vote for Quickbayt also. It comes in pellets, strips or spray. Kills flies in less than a minute and really works. I do not use it in the house (just a swatter but will try the dawn thing) but it works amazingly well outside and in the barns.