Can someone be truly brainwashed like we think? No. One of the important things God gave us is free will and God is really touchy about anyone(including Satan) messing with that.
What I think brainwashing is bringing bits of that nasty sin nature we are all stuck with to the surface. The nastiest parts that only God can normally see and knows about. Hitler and the Russian communists got at least 3/4 of their populations to hate on the Jews. Normally how people felt about Jews was buried deep within people. Then people like Hitler, Stalin, Lenin, they used propaganda and what we call 'brainwashing techniques' to bring that nastiness to the surface for all the world to see.