Vayikra (Leviticus) 19:31

Al-tichnu el-ha'ovot ve'el-hayid'onim al-tevakshu letom'ah vahem ani YHVH Eloheychem.
Don't turn to those who practice ov, nor to the wizards. Don't seek them out, to be defiled by them. I am YHVH Your Elohim.

Ov is a specific type of idolatry that involves communicating with the dead. To be involved with this will defile us, because YHVH is our Elohim, not other spiritual forces.

There shall not be found with you anyone who makes his son or his daughter to pass through the fire, one who uses divination, one who practices sorcery, or an enchanter, or a sorcerer, or a charmer, or a consulter with a familiar spirit, or a wizard, or a necromancer. For whoever does these things is an abomination to YHVH: and because of these abominations YHVH your Elohim does drive them out from before you. You shall be perfect with YHVH your Elohim. For these nations, that you shall dispossess, listen to those who practice sorcery, and to diviners; but as for you, YHVH your Elohim has not allowed you so to do. (Devarim/Deuteronomy 18:10-14)

Another reason YHVH doesn't want us to be involved with this is the very reason that the nations of Kena'an were driven out. He calls them abominations.

King Sha'ul errs in this sin when he seeks out the medium of En-Dor to communicate with the prophet Sh'mu'el (Samuel) (1 Sh'mu'el / Samuel 28)

1 Divre HaYamin (Chronicles) 10:13 
So Sha'ul died for his trespass which he committed against YHVH, because of the word of YHVH, which he didn't keep; and also because he asked counsel of one who had a familiar spirit, to inquire.

For one thing, physical contact with the dead makes one ritually unclean. Therefore how much more does spiritual contact with the dead defile us?

Another thing, YHVH is the one we could inquire of for counsel.

Mishle (Proverbs) 16:1 
The plans of the heart belong to man, but the answer of the tongue is from YHVH.

Shalom v'brakhot
Moreh Chizkiyah Shlomo
http://www.frontiernet.com/~toddlerville/home.html

Be libi tzavanti imratecha le ma'an lo echta lach.
(Tehillim 119:11)