NFHS Mechanic for Flagrant?

[Fifth And Goal]

Is there an approved mechanic for signaling a flagrant personal foul? I had this situation come up last weekend in a boys h.s. rec league game I was working. I am trail in 2-whistle, and A1 is dribbling at the top of the key. He makes a move to drive to the paint, and B1 steps over from the elbow, takes two hands and shoves A1 violently to the floor.

Here's what I did. I blew the whistle and went up with the closed fist, then brought the intentional signal. I had decided that the contact fit the definition of flagrant (i.e. violent enough to potentially cause injury or instigate a fight), so I decided that B1 was done. I simply told B1 that he was disqualified, and told his coach the same.

My question: is there an NFHS mechanic for this, other that the "shotgun?" And is the "shotgun" even an approved mechanic?

BTW, B1 and coach were not happy about it, as they only had five players, and they had to finish the game with four.