There are multiple ways you can disable a Gunk Spitter. 1. Aim towards it, and hold down the vacuum button on your glove. Catch the projectile it fires it and shoot it back to stun it for a few seconds. 2. Get near it, vacuum it and pull its head off. This will destroy it and it'll no longer bother you. 3. Fire your Pulse Cannon at the Gunk Spitter to stun it temporarily.