You are absolutely right to be concerned about those involved in Wicca. They are heading down the path of their own destruction, and they may lead others astray also.
Here are some of the Biblical texts which I would suggest you consider:
Deuteronomy 13 (whole chapter)
1 John 2:18-29; 4:1-6
I also found a couple of articles on the Web. The first two are by Probe Ministries (a sound organization).
This is the Spiritual Counterfeits Project Website, and their Spring/Summer 2000 issue of their magazine focuses on witchcraft.
This is a deadly and false religion, which is in no way compatible with the gospel or Christianity. I would encourage you to warn those who are into this error, and to guard the believers from being infiltrated by those who hold these views.
I just looked at CBC bookstore on-line, and a fellow I used to work with — Ken Boa — has a book which looks like it would be of help:
Cults, World Religions and the Occult
By: Kenneth Boa
Type: Trade Paperback
Vendor: Cook Communications
CBD Stock Number: WW6938239
Dimensions: 8 1/2 X 5 1/2 X 1/2
Retail Price: $9.99
CBD Price: $7.95
Our prayers are with you. This is indeed a most serious issue.