The most effective way I've found to deal with wasps is a spray bottle containing a mix of water and a generous amount of dish soap. A direct hit will initially make them incapable of flying, and the soap kills them within minutes. Err on the side of caution and hit the fucker a second time once it's been immobilized.

It's also useful for destroying wasp nests in the event they're built somewhere that you don't want to use toxic chemicals. It may require a couple soakings over a day or two before it ruins the nest, but it works.

Note: Wasps get mad as fuck at everything. Warmer temperatures make them more aggressive too. Additionally, dead wasp bodies release a scent that other wasps interpret as being in danger -- this, too, makes them mad as fuck.