You know, I don't think you can actually do that.
I'd never considered the possibility that someone would want to prevent equipping a shield with certain offhand weapons, and even the tags that prevent a weapon from being equipped with specific other wtypes doesn't include locking out shields for that wtype.
I thought just now that maybe you could workaround this by disallowing the shield armor via the traits box on the claw weapons, but it turns out you can only allow equipment from that box, not disallow it.
I'll have to add this function in the next update.
~Ramza