“Excuse me, would you be ever so kind to remove your cart from my colon? Before I shove something up yours?!”

Too funny! I might have turned around and told them that it was inappropriate touching of my butt by a foreign object in a loud voice and let them deal with people staring at them. Public embarrassment of them works very well in some cases. I did that once at a Walmart when I was in a similar situation as yours, and they left the line as if they forgot to add something to their shopping cart.