Rigidbody.useGravity  

// Disables gravity on all rigid bodies entering this collider.
function OnTriggerEnter (other : Collider) {
if (other.attachedRigidbody) {
other.attachedRigidbody.useGravity = false;
}
}
// Turn this collider into a trigger on startup
collider.isTrigger = true;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnTriggerEnter(Collider other) {
if (other.attachedRigidbody)
other.attachedRigidbody.useGravity = false;

}
void Awake() {
collider.isTrigger = true;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnTriggerEnter(other as Collider):
if other.attachedRigidbody:
other.attachedRigidbody.useGravity = false

def Awake():
collider.isTrigger = true