My dog used to do that. I was shown two methods for stopping it. First is to turn around and walk away just as the dog gets back to you and call the dog into heel. After a few paces keep walking but bend down and take the dummy from her mouth and say "dead", then heap the praise on her.
The second was to let the dog come in but when she drops it say nothing (i.e. no praise) but calmly pick the dummy up and gently but firmly put it back in her mouth with the command "hold". Now while she's holding it and sitting gently stroke her chest repeating "hold" and "good girl". When you want her to