Khloe Kardashian and Tristan Thompson are threatening to take a woman to court for continuing to claim Tristan's the father of her child -- and they say DNA proves he's not.
The lawyer for the co-parenting exes -- pit bull attorney Marty Singer -- fired off a cease-and-desist letter to Kimberly Alexander, saying ... "We demand that you immediately stop defaming them with malicious defamatory lies and specious fabrications" about the paternity of her child.
According to the letter ... Tristan had previously taken a paternity test in which it was "unequivocally proven" he's not Alexander's child's father, but she's continued to spread lies about Khloe and the NBA star, including claims they falsified or "fixed" the test.