I'm not training dressage, but I have this very problem at the trot with my lesson horse.
My trainer tells me to think of my core as a tree, and my arms as tree branches. The horse can pull and move the branches, but the tree does not bend...and eventually, the branches come right back up to their natural level, bringing the horse along with it.
Of course, in the ring, this is more like...
Horse: Hahahaha. I will now seize the reins! <yank>
Me: (*@%^!&! STOP THAT.
Trainer: Don't forget! Be the tree!
The image really has helped me a lot, though.
Another thing - I don't know if this is an issue for you, but just in case - are you making sure to keep your hands really quiet? My lesson horse will sometimes start this nonsense up with no provocation at all, drat him, but he will almost *always* do it if my hands are less than silent.