Spraying with WD40 or Plusgas won't make any difference.
Here's how to do it:
Turn the bike upside-down and get it stable. Work out which direction is turning towards the back of the bike. On the LH side that's clockwise and on the RH that's anti-clockwise. Place the spanner on the flats so that it's as close as possible to the crank then put pressure on it so that the force goes "out and back" and the leverage of the crank counteracts the leverage on the spanner. Be careful you don't stab your self on the chainring because to remove the drive side pedal you will be setting the spanner in a rearwards direction. Push down like hell and if necessary tap sharply with a wooden or rubber mallet. These pics might help explain, even though in pic 1 I'm actually tightening:
If hand pressure won't work, set the bike upright as in pic 2, stand astride and actually stand on the spanner and bounce. It will go. In pic 2 I'm turning towards the back of the bike to undo.