How do I really disable comments?

I've been on a wordpress hiatus for the past 8 months and have never had to deal with building a website for an actual client. Though, no matter what I tried/how I tried to disable comments, I would always get spammers commenting. What can I do to make sure that comments are really disabled... and what can I do if I want to enable them, but spam gets too much. In essence... what is the industry standard to deal with this?