Yep, what Sara says...
I have had a dog who did this

I would wait untill he was finished, smile at him and say: thank you, now, what I want you to do is....
Actually I love dogs that do this. It shows they really want to work, although they are impatient and pull open their trickbox as soon as they see a treat.
I have been teaching Jinx to speak, now she thinks, every time I ask her to sit, she needs to speak

I don't reward this anymore(I did when I first started training this) Now I ignore it, because I only want her to do it on cue.