There aren't global settings for everything (I think the number of settings is confusing enough as it is, and it would get out of hand if I tried to make everything configurable). You could change the default behaviour by modifying the database schema files though (bearing in mind that you might have to re-do your changes after upgrading nBill in future).
For example, to make products non-taxable by default, you could edit /administrator/components/com_nbill/framework/database/schema/product.xml, find the column definition for is_taxable, and change the default value from 1 to 0.
