Sure they are. Judas- 'the son of perdition'- is a clear example of this in the Bible. He invited Satan into his heart, he didn't accidentally pick up a demon. Then he wanted to make money off of the death of Christ for satan. He was never forgiven, and was replaced by Matthias. Also, we are told about the end times sellouts, who are called 'the children of perdition' 'and disobedience'- that they too have a spirit working in them (Satan) and to not have anything to do with with them. I don't know what Bible you are reading that says one day you can be with Satan, and the next day with God. But it's not true. There are actual sell outs who cannot be saved. Once a person blasphemes the spirit of God like Satan does, it's a literal game over for salvation for them. Then there is also Zechariah 11 in the Tanakh..