The snake-person is humanoid down to the thighs. Below the thighs they have a pair of snakes replacing their legs. Specifically, the snake's spine attaches with the last thoracic vertebra being jointed to the lower part of the femur, which has the form of a snake's vertebra rather than its usual form in humans. The musculature blends between the serpentine and human sections. These snakes are quite a bit longer than human legs, and are very flexible. The cloaca on the back of the leg and no serpentine reproductive organs. They are terrestrial, and move over land by slithering with their snake-legs

Because this is a different species with different anatomy, the methods used to bind mermaids will not work on the snake-people

How could you use rope to tie up their snake-legs, so that they couldn't get away or attack easily?