Unfortunately for the US finding some legal means to take out WikiLeaks is for the most part an impossible task. Attempting to use some sort of espionage legislation is likely to fall flat on its face for a variety of reasons.
To visualize this issue in a legal way you must first visualize a long chain with one link for every man woman and child on the planet. All the links on the chain are painted green except for the first link that is painted red. The red link belongs to the person that actually accessed the US data base and made copies of files. Under the US law, that person is the only person who violated the law. That person will be subject to the full weight of the law and possibly will receive the death penalty.
In the broadest interpretation, all the green links of the chain are a victim of a crime perpetrated by the red link. Even that the first green link is WikiLeaks, it is still a green link. This is comparable to getting the flu. The green links were exposed to the red link who was the carrier, so to blame the green links for getting sick and missing work is for the most part protected by labor laws. An irate employer in need of anger management was the reason that law was put in place.
So as each green link is essentially a victim, it is both immoral and unethical to go after them. But then again we see this all the time in some civil case when the lawyer casts the broadest net they possibly can often if not always netting innocent people in the attempt to recover damages. A neighbor may be netted just because they lived or live near a person being pursued.
So in the case of WikiLeaks, they are just the first tier of people to receive this now public information. They are not blackmailing the US government looking for some sort of ransom to be silent, their motives are very different and not self serving. Even that the Wikileaks founder has set a data release time bomb, it is others that are keeping him from accessing that timer. So if the release happens, it is difficult to say who is to blame the release happened.
If the US created a law or enacts a old law from a different time, they essentially will be butting heads with the very first law and principal the US was founded on, and that is freedom of speech and expression. WikiLeaks is under no obligation to be silent, and the attempts to do so by secondary means simply underscores that reality. The sexual assault charges are just such an example. As yourself how often INTERPOL list alleged sex offenders of the caliber alleged on their red hot list, and you will find all the proof and convincing you need.
So if the US goes after the first green link, then they also must go after all the other green links in the chain. There is no difference between the links, and any attempt to created legal a difference would not stand up in court and clearly violate the first amendment of the US constitution.