How can I fix a tub spout that has some running water when the shower is on? It's not a lot but when you pull the knob to turn on the shower you can see that there is still a little be of water running from the spout.

I current have a MOEN Banbury Single Handle Tub and Shower Faucet in Spot Resist Brushed Nickel (Model # 82910SRN) installed.

Thanks,
Ennio