maxScore = 0;
  setInterval( function() {
  	if (score > maxScore) {
  		maxScore = score;
  	}
  	if (distance(hero.y, hero.x, ennemi.y, ennemi.x) < (vitesseEnnemi * 15 + 30)) {
  		hero.y += (hero.y - ennemi.y) * speedLimit / distance(hero.y, hero.x, ennemi.y, ennemi.x);
  		hero.x += (hero.x - ennemi.x) * speedLimit / distance(hero.y, hero.x, ennemi.y, ennemi.x);
  	}
  	if (spawn > PowerUpspawnRate) {
  		hero.y += (powerUp.y - hero.y) * speedLimit / distance(hero.y, hero.x, powerUp.y, powerUp.x);
  		hero.x += (powerUp.x - hero.x) * speedLimit/ distance(hero.y, hero.x, powerUp.y, powerUp.x);
  	} else {
  		hero.y += (reward.y - hero.y) * speedLimit / distance(hero.y, hero.x, reward.y, reward.x);
  		hero.x += (reward.x - hero.x) * speedLimit/ distance(hero.y, hero.x, reward.y, reward.x);
  	}
  }, 25);

If you want your green guy to flat-out avoid the enemy, just wrap the second set of conditions there in an else block.

edit: Formatting for readability/copy+paste

I think I need to get more fresh air or something.