I need a frugal method to kill flies

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We had some friends over for a BBQ and with people going in and out, I now how a bunch of flies in the house. I tried setting out some vinegar with a little dish detergent but didn't catch a single fly. I even bought a pest strip but the flies seem to avoid it. The only flies I've caught so far are the ones that have landed in my cup of coffee! EWWWWW! Does anyone have any mothods they use (other than a fly swatter - I don't seem to be very good at that either).