You cannot block people from seeing your site, and even if you could block them as an email subscriber, they could just sign up through a different email address, or on a wi-fi network. And besides, IP addresses are no longer unique. Literally hundreds of users can be assigned to the same IP address. This is done by ISPs to keep from having to buy new blocks of IP addresses, which cost $$ and also because there is a physical limit on the total number of IP addresses possible.
Trying to block someone from visiting your site is sort of like trying to keep someone from walking down the street and looking at your house.
You can put their email address, username and if they provide it, a website into the "blacklist" at settings > discussion, which will keep them from commenting - at least until they figure out you are blocking them, then they will probably figure out to use a fake email and username. If you put their IP address into the blacklist, you could potentially be blocking hundreds of innocent people from commenting on your blog (actually they go into moderation, they aren't truly blocked.
If you want to block them from seeing an image or an item, either set that post to private, or set a password for it. Passwords aren't very secure though as all it takes is someone putting it on facebook or twitter, or someone passing it to friends to thwart a password.