If that thing really is the foundation of the Power of the Titans and they can get it to manifest again by killing Eren, they could somehow interfere with it and remove it from him, separating it from the power it created.
Without the source of the Power of the Titans, Eldians would no longer be linked through the paths. The Nine Titans could lose their powers and the Curse of Ymir. The Wall Titans across the world could lose their forms, ending the Rumbling all at once. They might even return to their old human bodies, and Connie’s mother too for that matter.
This is how the Power of the Titans will be destroyed forever.