The indefinite pronoun anyone (one word) refers to any person but not to particular individuals.
Any one (two words) is an adjective phrase that refers to specific but unidentified things or individuals.
A similar distinction applies to anybody and any body, nobody and no body.
The enemy is anyone who's going to get you killed, no matter which side he's on. Any one of your buddies, if he's careless enough, could turn out to be your enemy.
(a) Does ______ know who first said, "You can't trust anybody over 30"?
(b) If ______ of the 25 barons should die, the remaining barons shall choose a replacement.