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You need a good ball-peen hammer for the job. The impact of the hammer needs to distort the taper of the arm (the part you're hitting) in order to pop it off the balljoint's shaft. The smaller contact point will require significantly less effort (and do less damage) to achieve this than a mini-sledge or club hammer.

On a Datsun strut front end, this process can damage the balljoint itself if the steering arm is not held securely, either by reconnecting the strut, or by removing the assembly and holding the arm in a vice. But it is unlikely you'd need to remove the arm from the balljoint unless you are replacing the balljoint.