How do I remove stains from teak? Is oiling or sealing advisable?

Believe it or not you can try what caused the stain in the first place, water. The water moved some pigments in the wood as it soaked in and the stain is the kine wher the pigments stopped moving. Add some water to a towel or sponge and rub the area, allowing some soaking from time to time. Do not oil until your are satisfied with the stain, the oil will set the stain. You can also experiment with putting an old shirt over the stain and use your steam iron to steam the stain. Once you are happy I'd brush or spray some Thompson's® WaterSeal® Advanced Natural Wood Protector - ThompsonsWaterSeal.com