You just go in and arrest him at the embassy. I mean - he's in London, we just go in and take him.
Wait, did you say that the Ecuadorian Embassy is actually sovereign land and to send a police or military force in to arrest and remove him would be an act of war? Well, you don't need to worry about that. We've just proven, by way of 59 cruise missiles, that even sovereign nations who do bad things are no barrier to the will (or should I say whim) of the United States. And they don't even have to go in by hand - I think a targeted drone strike would have a limited number of civilian casualties. And London doesn't have any room to complain, since they were perfectly fine with all the drone strikes in middle eastern countries where there were known criminals and we (usually) limited the civilian casualties.
I don't see how this is going to be difficult - the US just needs to apply traditional tactics used on physical terrorists to the new crop of information terrorists.