Sodium channels are specifically blocked by:

Another means for studying the flow of ions through an individual type of channel is to block one type of channel at a time. For instance, the sodium channels can be blocked by a toxin called tetrodotoxin when  it  is  applied  to  the outside of the cell membrane where the sodium activation gates are located. Conversely, tetraethylammonium ion blocks the potassium channels when it is applied to the interior of the nerve fiber. Key Concept: Tetrodotoxin (TTX), one of the most potent poisons known, specifically blocks the Na+ channel. TTX binds to the extracellular side of the sodium channel. Reference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no 67