I understand one of our Bishops said, “Always offer for the former pastor to come back. Former pastor, always refuse.”
While having a denominational set of rules isn’t everyone’s cup of tea, and I don’t always like every rule in my denomination, this is one of those places where I like there being rules about one pastor interfering with another pastor’s work. Retired pastors who do some of the things Dave has said above in the UMC get reported to their Superintented and it gets squashed pretty quickly. The denominational leader can be the heavy and take the burden off the pastor.
How you put a stop to this’s when pastors are autonomous and congregations are as well, I have no idea. But it caused me a lot of trouble in my last Baptist congregation when the former pastor who served 20+ years was invited to nearly every funeral I ever did. He also routinely interfered in my pastorate. But those are stories for other times.