How do I stop people from going around me directly to my dad after the handoff?

Refuse the end-run, every time, without exception, with the founder's full backing. Kris Kluver, in The Dysfunctional Family Office, has Gail Mitchell make this her biggest concern with her father at the handoff. If anyone tries to go around her to him, he brings them back to her. They establish the behavior right out of the gate. Without the discipline, the team's old muscle memory reasserts itself within weeks. Brett the CFO calls Dad about the credit line. The biggest client calls Dad to discuss strategy. Frank the attorney clears decisions with Dad. Each end-run, individually small, accumulates into a pattern that erases the new CEO's authority. The founder has to be willing to redirect every call back to the new CEO, even when it's awkward, even when he could just answer the question himself. The discipline takes about six months to set. Kris Kluver works with founders and new CEOs on enforcing this exact rule at thethirtyadvisors.com.