Multi wound models vs multi wound attacks

It is about Wounds on the profile. Ogres have 3W on their profile. In the case of multi-wound units, spare wounds inflicted are not on any specific model, but on the unit as a whole. You "determine how many wounds are caused on each model individually (remember that each model cannot suffer more wounds than it has on its profile). Add up all wounds caused on the unit and then remove the appropriate number of models, noting any spare wounds on the unit"  (BRB p. 45).

So, the Tree Wack causes 6 wounds on the Ogre, which is capped at 3, and then you add any spare wounds already inflicted, in this case 2. The end result is that 1 Ogre dies, and there are still 2 spare wounds on the unit.