Yes, "stopping the clock" with either a fist or open palm is something JD is very anal about.

In the 2016-17 CCA manual JD mentions it specifically in the Q&A section near the front, and again on Page 77: "When an official calls a violation: Simultaneously give a sharp blast of the whistle, stop the clock using straight arm and open hand, then drop the whistle from the mouth."

Interestingly JD isn't a huge fan of two-handed reporting yet it became mandatory last year.