Basically, they fixed it to a point...if you initiate a trade just as someone starts teleporting, it doesn't work. However, if you initiate the trade right as they are about to finish it will go through and block them. My friends and I troll one another this way all the time.

Prior to that update, you could interrupt someone's teleport at any point. The timing is tighter now, but it's still possible to do. So, it was never really fixed at all.