Removing a Planeswalker

Jack and I were playing Magic at the Pokemon X&Y release event. 

Jack cast Ajani, Caller of the Pride

I had reacted at Instant speed with two Spark Jolts

and a Magma Jet

assigning all damage to the Player Ajani.

Jacked rightfully questioned this play. 

We ruled that this was not legal since Ajani is not a creature & not a player.

Is this the correct ruling?

Is there a general guide to Planeswalker removal?