I typed this command in a cmdBlock :

/summon Witch ~ ~1 ~ {AttributeModifiers:[{Name:generic.movementSpeed,Base:0.0}]}

I don't think it's wrong, so I don't know why the Witch still moves instead of being still...
About this one, I'm less sure it has the right syntax :

/give @p 276 1 0 {AttributeModifiers:[{Name:generic.attackDamage,Base:8.0}]}

But same happens, Attributes seems to be not working...