There are certain skills that can't be blocked or evaded. I think Seeking Blade is one of them. Most of the time, you can't really do much to someone using stances to block your attacks other than wait until the stance ends or use different skills that can't be blocked or evaded like hexes.
Either bring skills that take effect regardless of whether or not you hit, wait for the stance to end (usually not an option for wards), forcefully end the stance using Wild Blow (not an option for wards) or negate the evading/blocking effect using Rigor Mortis or Warrior's Cunning (gives a short window of opportunity to finish the job).