Ok, maybe "Banned" was the wrong word to use.
From my understanding (which may or may not be correct, this is what I was told), they were regularly flying out of NCL with less than half of the bags on board. The Airport and Servisair put a LOT of pressure on them to either make a tech stop, or reduce their passenger loads.
Servisair threatened to cancel the contract, which would have left them without a handling agent, as Swissport were not prepared to handle them either. This effectively would have blacklisted the company from the airport I imagine.
In the end, I think some of their flights did make refuelling stops just to keep the peace. Still by the looks of things, Viking have made a lot of changes since then, perhaps NCL might welcome them back!
RTG!