If we are talking about a freewheel, and not a cassette (a freewheel screws on to the hub - while a cassette drops down on to the hub and is held in place by a lockring), You could try using a "cheater-bar" - a long length of pipe that fits over the handle of the wrench. This gives you more leverage to apply to the recalcitrant freewheel.
Some mechanics have been known to stand on the wrench to apply more force - but this can cause damage.
Good Luck!