Our German Shepherd puppy has learned to push his ex-pen around on our floor, how do we handle this?

Hi Cindy,
I have an 8 week old purebred German Shepherd puppy. We've had the puppy for almost a week now and he is pretty OK to spend time playing in his ex-pen when he is inside and not on leash (this is usually after we have been playing outside for a while trying to blow off some steam). Usually for a minute or two when he is first put in the ex-pen he will have a bit of a short tantrum before flopping down and taking a nap or chewing one of his toys. We try to ignore him and not give him attention of any kind when he is having his tantrum as we don't want to encourage it. He doesn't try to climb up the ex-pen, and if he stands up and puts his front paws on it we give a firm 'NO' and poke him down as recommended in your management course. However, our home has vinyl plank flooring, and lately while having his tantrums he has realized that he is able to push the ex-pen around on the floor, which we obviously do not want. We are a bit torn on how to handle this.