http://www.nurcin.co/unity-c-disable-jumping-more-than-once/

PlayerController.cs

private bool isgrounded = true; //let's assume he's on the ground
void oncollisionstay (collision other)
{
	if (other.name != "ground") //let's assume the name of the ground object is ground
{
isgrounded = false;
if (isgrounded = false && other.name != "ground")
{
//disable the jumping and wait for when the player is on the ground again
	if (other.name == "ground")
	{
		isgrounded = true; //add the jumping mechanic back in
	}
}