You can't assassinate the vikings out of the game, so you will have to get your bishops in there early to start converting their main province of Hordaland(sp) to stop the annoying raidinng. Once Hordaland is converted to christian the Viking leader will convert his faction. This will turn pagan Jutland into a very rebellious province. Your spies can do the rest. Once they've been wiped out and become rebels, you can bribe the rebels and keep a token force there to maintain loyalty. Due to the region's isolation it will play no more part in your campaign.