How do I block a number on my iPhone, and will the person know they've been blocked?

Easiest way: open the Phone app > Recents > tap the little 'i' next to the number > scroll down > Block this Caller. For a contact, open the contact card and do the same. For texts, open the thread > tap the name at the top > Info > Block this Caller. The blocked person is never told. Their calls go straight to voicemail (you won't be notified), and their texts simply don't deliver to you, though they still see them as 'sent' on their end. No alert, just silence.

The 'no notification' part is the reassuring bit. To them, calls go to voicemail and texts look sent but you never get them. Nobody gets a 'you've been blocked' message.

To manage your whole list: Settings > Phone > Blocked Contacts, and Settings > Messages > Blocked. You can add and remove numbers there too.

For spam specifically, also turn on Settings > Phone > Silence Unknown Callers. It sends anyone not in your contacts straight to voicemail without blocking them one by one.