I've heard that you can roll them back and forth in your hands a little, and that might encourage them to unroll. I've never had to try much though, Lily balls up very rarely, and when she does, she comes out of it after just a couple of rolls back and forth.
Shinjy's only ever rolled up into a ball completely once, and I just left him on my lap and he soon unrolled on his own. That was when I had first gotten him. Since then, he'd rarely roll into a ball, and when I flip him onto his back, he struggles immediately to unroll, so he can.........sleep! :lol:
Usually, the rolling back and forth does it, or just complete calm. Since it's usually a defence mechanism, when things are quiet, they will unroll so they can amble around or even find somewhere else to hide.
Satin doesn't ball up often, but when she does, it's because she's upset. What I do then is be very still and talk to her softly. I stop petting her, just hold her in my hand (tummy side down), and tell her she's a good hedgie and I'm very sorry about whatever happened that made her feel like balling up. After awhile, she'll stretch back out.
If Hector is balling up a lot, I'd do everything I could to make his environment more to his liking - dim the lights, mae sure the room is warm, quiet the tv/radio/etc, hold him without moving much... You could also try a bribe like putting a mealie, kibble, or whatever he enjoys near him so he can smell it. And if you're getting desperate, you can hold him in your hand and put a small blanket, towel, or t-shirt over him so he feels covered and safe. Make sure not to leave him along with any of those things though.