This happened to me, it was a right pain in the ass. I had to cut through the bolt at every space to get teh arm out, then I had to drill the bolt out with cobalt drill bits. It took me 2 weekends of solid work.
This is caused by corrosion causing the hub carrier to expand, in my case, the person that fitted the powerflex bushes also did not use any copper slip so the stainless steel tubes in the bushes welded themselves to the bolt too.
Make sure you use plenty of copperslip when you reasemble and hit the hubs with a hammer a few times to get the rusk flakes off. Scrape out the hole the bolt goes into too.
Alfa GT 3.2 V6 - Daily Driver.
Alfa 145 QV ph1 in green (Death by rust project) For Sale.
Alfa GTV TS ph2 in blue