I have friends who are Seventh Day Adventist and in the Unification Church. One believes in soul sleep and the other believes we can communicate with dead relatives. Are there any biblical passages to refute their beliefs?
Consulting the dead is clearly condemned in Deuteronomy 18:11. We also see Saul attempting to do this just before his death in 1 Samuel chapter 28.
As to the matter of soul sleep, I would look at texts like these:
The Bible teaches that there everyone who dies will be raised from the dead, either to everlasting bliss (heaven), or to everlasting torment (hell).
You may also wish to look at these lessons on the BSF Website: