I treated my previous Spider to new rear discs/original Alfa pads. The handbrake wasn't bad before but was definitely better after. Rear discs on Spiders have a tendency to get quite rusty and grooved from periods sitting unused. It will take ages for new pads to bed in on grooved discs.
My current Spider appears to be suffering a similar fate but to be honest, I've always had good handbrake performance from all my cars (after my non-Turbo Uno models, they were awful), so actually, this reply might not be as helpful as it first appears
So let's try again. New genuine pads (likely to be softer than 'performance' replacements, and therefore work better for what you need). Then, loosen the lever nut right off so the cables are slack. Wind back the pistons, fit the pads (and discs if appropriate), then reassemble and reapply the foot brake many times. Leave the handbrake cable slack for this. I've seen it happen many times that people over tighten the cables - then the adjusters can't self-adjust correctly. Also, over-tight cables make the lever harder to pull up and therefore less effective.
Obviously you need some tension so the lever doesn't reach the end of its travel, but just be careful not to overdo it.
At least five clicks should be easy to apply, seven at a strong pull.