Is genocide ever morally acceptable?

Genocide is the wholesale killing of every man, woman and child in a cultural or ethnic group. Ethnic cleansing is a euphemism for genocide, but I think it sort of gets to the heart of the matter. The goal of genocide is not just to kill people, it is to eradicate an entire culture or way of life that you think, for whatever reason, is unacceptable.

I don’t believe genocide is ever morally acceptable. But I’ve talked to several religious people (some on Fluther) who defend the practice of genocide in the Bible (the only religious text I know of to actually command the practice). In Deuteronomy 20:16, God commands the Hebrews to kill every single person living in the holy land, so that their culture does not survive to pollute the purity of the Hebrews’ religion. In the book of Joshua, this genocide takes place, to great celebration. In the books following Joshua, God punishes Hebrew leaders when they are not complete in their genocide of holy land cities. Deuteronomy 13:12 commands you to wipe out of any Hebrew-controlled holy land city that relapses into previous religions. When—and why—are such practices morally acceptable?