Should We Not Say Anything Against Voodoo?

Questions about how to respond to someone who thinks we shouldn’t say anything against Voodoo since it’s “just their culture” and arguments to refute a proponent of the African traditional religion as a means of worshiping the almighty God.  

• When I expressed concern about a movie dealing with Voodoo magic, one of the students said that it’s “just their culture,” as if we shouldn’t say anything against voodoo since it’s a cultural practice. How should I have responded?
• What arguments would you present as a polemic to refute a proponent of the African traditional religion as a means of worshiping the almighty God?