I've honestly never had a bystander butt into a conversation that I'm having with a friend or acquaintance and tell me to "spit it out" or anything like that. I feel like my gut reply would be like "you have no idea about anything" just because "spit it out" is a simplistic and frankly terrible solution to stuttering. It's a neurological problem and we don't solve it by "spitting it out." Put them on the spot and ask them for a solution.