Well, time travel is one thing, but for me what works best is a thing called P.E.S.T.- or Portable Electro Shock Therapy. When a rabid clapper/whistler does it more than once in a 2 minute period, just move in behind them and WHAMMO! they will be as docile as a kitten for the rest of the show. I think Grace Designs makes a nice lightweight but powerful model...
Seriously, though, I haven't done it yet but a whistle has a very specific frequency and you can highlight it, open your parametric EQ and create a very tight band notch filter on that frequency. Use the "preview" button and play with the parameters- frequency, bandwidth, attenuation- until it is minimized.
Worst banter offender: Sam Bush (he can be funny, too, but some major groaners at every show)