A ball joint removal tool makes short work of removing the ball joint. The tool can also save the stock ball joints and grease-filled boot, which in this case were still in good condition.
The constant force applied to the ball joint by the tool will pop it loose and won’t damage threads or other components that might come with trying to hammer them loose.
Remove the bolt that holds the top of the ball joint in the steering knuckle.
You can use a small pry bar or a sturdy flat-blade screwdriver to pry apart the knuckle to release the ball joint from the steering knuckle. This model of Subaru Impreza, the STi, uses alloy sleeve inserts that fit over the steel tapered stud on the ball joint to adapt them to the aluminum lower control arms. Make sure to transfer them over to the Whiteline ball joints. You can apply anti-seize on the ball joint to aid in removing them at a later time. It prevents the ball joint from rusting into the steering knuckle as well. Installing the ball joint required some prying. A floor jack can help ush the control arm into position, which allows you to insert the bolt to hold it into place.