Is drinking alcohol without soda haram or halal, and is a small amount of it permissible?

Removing soda from wine does not change the fact that it is an intoxicant, and therefore does not alter the ruling of its prohibition. Your friends' conclusion that a small amount of it does not intoxicate, and thus is not forbidden, is an incorrect conclusion, for "that which intoxicates in large quantities, a small quantity of it is forbidden." It has been authentically narrated from the Prophet, peace and blessings be upon him, "Everything that intoxicates is forbidden, and if a 'faraq' (a measure) of it intoxicates, then a palmful of it is forbidden." And in another narration, "Even a sip of it is forbidden." Therefore, you must warn your friends against this evil.