I'm using poisoned blade, gives a character stealth, and when he bypasses another character with stealth, the defender pays 1 gold, 1 influence or discards the character. Am i able to stealth my enemies characters that are already knelt? Stealth ability doesn't mention anything about the character being able to defend, just any character the defending player controls

Sure, as you say, there is no restriction on Stealth needing to target standing characters.