How can I stop a remote user from accessing my Kitkat 4.4 tablet?

[AC Question]

I am not very good with computers. My son has a Kitkat tablet and he is 8. He told me tonight that half dressed women are popping up on it. After looking at it, I have found that when I try to turn off wifi, it gets turned back on. I have tried a factory reboot and it does nothing. Icons start popping up at the top left corner of the screen. I am not sure what app Ceroa and Deroa are. I don't know how to get control of this tablet again. Can anyone help?