It is generally hard to prove a negative. AFAIK, there is no way to completely rule out R+L=J. The only recourse is to look at arguments supporting the claim that Ned Stark is indeed Jon's father. And those arguments have been summarized in the question you've cited:
Reasons to affirm that Eddard is Jon's father:
In the aforementioned investigation by Catelyn, a recurring rumor was that the mother was Ashara Dayne. It is implied by other
flashbacks (principally the story of the Knight of the Laughing Tree)
that Ashara and Eddard shared a mutual attraction.
When pressed by Robert, Eddard claims the mother is a servant girl named Wylla. Edric Dayne claims to Arya Stark that Wylla was his wet
nurse (making him Jon's milk brother), giving more credence to the
story, and that Wylla was not just a name Eddard made up to shut