The best thing to do is not to leave alcohol inside decanters for a long time as the alcohol will stain the glass and leave a ring mark.

To remove the stains from decanters:

• Pour a little white vinegar into the decanter
• Add some dry rice (as rice can act as abrasive) or decanter beads
• Swirl around
• Rinse the contents out the decanter thoroughly
• Wash and rinse the decanter in plain water with a dash of vinegar in it
• Make the rinsing water a bit hotter to evaporate the moisture inside the decanter
• Turn the decanter upside down to dry.