Can Defy or Windwall counter play abilities like Shieldbearer's stun or Harnessed Dragon's kill effect?

Ruling: No, Defy and Windwall cannot counter play abilities because they specifically counter spells, and play abilities are not spells.

Nuances:

• To prevent kill effects from play abilities, you can use replacement effects like The Boss or Zhonya's, or move the unit to a different zone with effects like Retreat
• If a play ability has a target restriction (e.g. "a unit at a battlefield"), you can cause it to whiff by making the unit no longer a legal target
• Harnessed Dragon's kill ability does not have the "at a battlefield" restriction, so moving the unit out of the battlefield won't help