The OP feels like telling her aunt to buzz off, presumably in one way or another (and I agree, Aunt probably knows all about blocking, etc.), would be heartless. And she doesn't want to be heartless. (If she didn't care, she wouldn't be posting, she'd just tell her to buzz off/block her/unfriend her).
It sounds like although she has done some stuff in real life that is more than just annoying, her facebook behaviors are more of the fingernails-on-a-blackboard kind of thing. Like! Like! Like! My niece is wonderful! I get how it's irritating -- you feel like you're being hung on -- but really, there's no harm done. It's not like she is badmouthing her or making trouble. More like a pesky little sibling who wants to be around you. It could be a lot worse.
I think you will be happiest, OP, if you do nothing. You don't have to read her posts or comments if you don't want to. And Aunt's personality isn't going to change in any other area of life anyway.
This is the kind of thing where you can make it lots less annoying if you make it a game and give yourself prizes. Like, for every twenty pointless "likes," you get a candy. For a hundred, you get to go shoe shopping.