To do this there are a couple of settings that need to be in place first.
1) make sure the business rule B2CTAX is turned on. You will find this in the advanced settings page of your site. editAdvFields.wws?cfield=advset (add that to your store's url)
2) customers by default are taxable because the system is designed for 'b2c' which always requires a buyer to pay tax. If you want to override this setting at the customer level the customer must have his taxable field turned ON and they must have a taxid number. Use customer maintenance to update the customer. mainteditcust.wws
Here is a link to a video that will show you some additional configurations - http://screencast.com/t/spEHkpl9yI
