The problem with Varangian Guard are not their high stats(they are suppose to be ELITE Guard unit formed from high ranking AngloSaxons, Scandis and Rus warriors) but the fact that they are dirt cheap(they cost less than a mercenary Light Swordsman) and cost ZERO LEVY which make them spammable.
The right solution(within current game limits) would be increasing their recruitment cost(a lot - and price them similar to troops hired from mercenary camps) and maybe add gold upkeep for them(to make them less spammable).
Cheers
