Yes, I kill spontaneity. Mercilessly and gruesomely.
But in all seriousness, yeah, I'm not big on it. If someone gives me a last-minute idea, I'm almost definitely going to say no. I wish I didn't do that, but I do. I'm fine with spontaneity when I don't have important plans, though. One example...
Friend: Hey, wanna go to a party tonight?
Me: Sure! I'm available.
(My previous plan had been watching TV all evening. I was still a bit annoyed for the short notice, but my plans weren't that important and the new plan sounded more fun.)