Oddly enough, fighters seem to be their counter, or at least what I use.
It takes them a while due to the fighters low damage, but the dagger frigates do small damage back themselves.
Yes, they are an annoying unit, but the marauders (hostile, friendly and the ones you can build are all the same unit) were designed with the few, but strong, units approach as compared to most of the rest of the game which is many, but weaker, units. This causes them to win pretty much any 1v1 battle and it's intended that you have to swarm them to take them down.
Just ran into them in my current game, swarmed them with 206 fighters. Lost 15 fighters to destroy 23 dagger frigates. Mostly Mk II fighters but it works.
D.