According to your setup, it seems the voice chat audio is generated on the iPad, probably along wit other audio. This comes mixed over the headphones wire to the Elgato and then to your PC. It isn't possible to unmix the audio, once it is mixed. To have it not processed by the Egato, it must not be fed into the audio line in the first place. Deactivate it within your iPad. This way you are not able to hear it any more, unless you can make the iPad output the voice chat only to the iPad internal speaker but not to the headphones jack.