"If the USA could close down wikileaks and dispose of him without anyone noticing they no doubt would "
I wouldn't blame them for closing down WikiLeaks, but there is no evidence to suggest that they want to "dispose of" Assange, legally or otherwise.
There's a lot of nonsense talked about "WikiLeaks" (it isn't even a "wiki" for a start, so the very name is a fraud)
The vast majority of the stuff on there is confidential US diplomatic documents on all sorts of things, the sort of stuff which needs to be kept confidential despite what Assange claims. Otherwise the whole practice and principle of such communications would not be possible. And that would not help anyone. And most of the "embarrassing revelations" are not about the US at all, although they are the ones which got most of publicity at the start.
The real culprit was Bradley/Chelsea Manning. He/she has already been dealt with. It would be bizarre as sandy indicated for the US to pursue Assange anyway as he is not a US citizen. At the very most he's handling stolen property.
Of course Assange's supposed obsession with free speech and openness cools considerably when it comes to putting the spotlight on his own affairs.