How do you stop a male dog who is not fixed from sniffing at female dogs that are fixed?

They do what comes naturally, and also, when they smell each others behinds it's a way of identification and a quick run-down on the dogs personality traits. However, it isn't the best thing for pet owners to have to put up with. I have a female Bichon Frise and a cockapoo x terrier that is male and both of them have been fixed. I basically train him not to do this. When he starts now I quickly say in a soft, but stern voice and point a finger "NO!" He stops and that's the end of that. I haven't had to do this too many times. It takes persistence, but they get the message sooner or later. Marcy