Protect a Mac from all unwanted inbound/outbound traffic?

Question

A friend of mine asked if there is some sure way to protect a Mac from all unwanted traffic. He was esspecially wondering about applications accessing the web asking for the updates and other "behind your back" traffic. Is there an actual way of blocking such traffic?

Firewall should do the trick, but it still does not sound sure.

Answer 1

An app like Little Snitch should work.

Answer 2

The built-in firewall should be sufficient to do what you want, but you'll, I mean your friend, will find it a nuisance after a while unless you're accessing only a handful of sites.

From a Terminal shell prompt:

man pfctl