If you’ve consistently used the same font throughout, then see this thread for a solution. (If I weren’t typing this on my phone, I would have copied over the key points. )
That thread has a link to a knowledge base article that explains how to set your default formatting for new documents and then how to apply that to existing documents.
If you have different fonts sprinkled throughout your project, then that method probably isn’t the way to go, as it will reset your fonts too.
You may want to first try it on a copy of your project to make sure you’re happy with the results.
It’s my recollection that the line spacing selector seems to be generated based on the various line spacings in the project. (At least, that’s how it appears to work. I haven’t bothered to consult the manual on this. )
So if the project is set to one consistent line spacing, then eventually the other unused choices disappear from the dropdown. Again, that’s based on my memory of what happened when I had multiple spacings and what happened when I subsequently removed them. YMMV.
I guess I misunderstood the original post. I thought the poster was trying to make line spacing consistent across documents. If @Fredthulhu is trying to remove all the line spacing choices from the dropdown without changing the formatting of the documents, I don’t know how that would be done or whether it’s even possible.