Agreed. It's really annoying that others use Facebook for auth as well. Want Spotify? Nowadays you better have a Facebook account because that is how you sign in unless you were lucky enough to have acces to their beta prior to the partnership.
https://www.spotify.com/us/signup/
If you already have an account, I'm not sure there's a way to de-associate it, but you could probably delete the old account and create a new one.
But I already gave up on Spotify since the web player is buggy and full of memory leaks. But I'm sure their apps are fine.