The nitrate will remain sorta high until all of the ammonia is gone. you need to figure out what is causing the ammonia. Do you currently have any fish, or is it just the snails you mentioned earlier?
Assuming that nothing in your tank is dying off, you may just want to give it a little time to allow the bacteria colonies to grow to sufficient proportions to be able to process the ammonia and nitrate. If you are not inclined to wait, the only other thing I can think of other than continuing your weekly 10% water change is maybe running some really high grade carbon in your filter for a while. I don't know if it will necessarily remove the ammonia and nitrate, but it will help remove any dissolved organic material, which will break down into these components. If you do try the carbon, make sure it won't leach phospates into your tank or you will have a huge algae bloom as well. I personally use Kent Marine, and I don't have any complaints. Also, be sure to use a filter sock for the carbon. A protien skimmer will do about the same thing. I hope this helps and keep us posted.