Nothing, by itself, can block Avada Kedavera. Harry Potter is a living miracle, having survived two direct hits, and deflected it twice, but these events were only due to other magical circumstances, all of which extremely rare and unique events.
This answer begs the question. Then how did Harry successful deflect Voldemort's Avada Kedavra curses at Neville Longbottom after he beheaded Nagini and at Molly Weasley after she killed Bellatrix Lestrange?
There are 2 possible answers: that by sacrificing himself Harry was able to defend them against everything or that is WAS possible to block AK by Protego. The second option is supported by the fact that in Deathly Hallows Part 2 Harry blocked multiple Avada Kedavra shots and Bellatrix Lestrange was able to block Molly Weasley's Killing Curse.