There is a specific solvent for most finishes. Lacquer thinner works best on lacquer, and will also work on shellac and water based poly, just not quite as well.
The lacquer thinner is right on the edge of being unsafe to use because of inflammability so you really want to think about the circumstances and conditions when you are using it. Se if you can find days to work where the natural wind direction will blow the vapor and fumes right out of the house, for instance.
Really, think safety when using something like this and be sure to read, understand and follow the label direction. An older couple had an explosion and fire near here last year. They were using gasoline to clean something in their basement. He was severely burned and the house was a total loss. Please, be aware of the issues involved in deciding what solvent to choose. I really suggest Bob Flexner's book Understanding Wood Finishing. He has some good info on pages 142 and 143.