How do you stop yourself from getting too far off track from your original story idea?

I know it’s natural for a book to change a LOT throughout the writing and editing process, but at what point do you feel as though it’s a completely different story idea from what you started with? Do you just continue to evolve your story, or do you try to stick to your original theme or idea? Or do you take those new ideas and start a new project altogether?

I ask this because I could see how easily one could become sucked into a rabbit hole and the story would just never reach an end if it continues changing so drastically.