Teach her a cue word for sniffing, and occasionally, when she is giving you what you want, give her what she wants (a good long sniff). Dogs sniff for many reasons - first of all being that it is their sense of smell is their most important sense. Asking a dog not to sniff at all is sort of like telling a human they are not allowed to visually take in their surroundings. But dogs also sniff to relieve stress or indicate to another dog (or human) that they are not a threat. If you are fussing at her or "correcting" the slow sniffiness with your collar/leash, you may be making it worse. You can also work at teaching her to pay attention and more heavily reinforce the times when she is so she has a good alternative behavior.
One of my favorite games is Leslie Nelson's "Find it game". Drop a treat on the floor or ground and tell your dog to find it. They get to use their nose to do so. Then you wait quietly for them to turn back to you to see if you have more. As soon as the dog looks at you, mark that behavior (clicker - better/or marker word) and do the find it again. Pretty soon you'll have a dog who can't wait to seek out your face, and is learning good attention. When the dog is consistently and quickliy turning back to you, you can start adding in steps before you mark - but only mark if the dog is still focussed on you. If the dog loses focus, don't mark but go back to the beginning and build it up again.