Hi Amy,
 
Once Thor has gotten one of the females to poop, he needs a chance to sniff
her poop and maybe her butt.  Once he's done that, he should be a bit less
aggressive.
 
Grabbing a female and shaking her until she poops is normal behavior for a
male.  I've been told that he's trying to figure out whether she's in heat
or not.
 
If he's drawn more than a tiny bit of blood, you're right, it's a bit rough.
You might try bitter apple on the females' necks to slow him down a tad.
 
Thor might be even more aggressive if he's meeting the females on his
territory.
 
One of my males, Boomer, sometimes seems just relentless with new ferrets.
It helps a little if I pick him up and scruff him and shake him and hiss at
him, then let him back down.  He is apparently willing to think I'm the top
ferret.
 
Best of Luck!
Dennis
[Posted in FML issue 1668]