Teach them to "throw" underhanded.
This is exactly what I was going to say.
This is the #1 thing I start with...tell the girls we are throwing NOT bowling...as soon as they know it's NOT bowling then their natural throwing motion will take over.....also never ever doing wrist flips or 1/2 circle with wrist flips will also help to curb the bowling tendencies.I see people here say this a lot here, but in my experience, it's almost never true. I can only think of a single girl I've worked with who has had something resembling IR mechanics immediately. And none of the others I'm thinking of have had any pitching experience or coaching to give them the wrong ideas. 90%+ want to bowl the pitch and have to be taught IR mechanics.