Needed: Phase Converter Box

Balanced or unbalanced mic? If balanced, it's just a matter of swapping pins 2 and 3; you can get ready-built XLR adapters that do this, for about $25, from suppliers such as Markertek. If unbalanced, a bit more work involved.

Or, you could just reverse the speaker wires.

Note that depending on what you're trying to do, it is unlikely to work: sound travels at about 1msec/foot in air, so unless your mic is right next to your loudspeaker you get comb filtering rather than actual rejection, and you'll actually get boost at some frequencies.