IF someone insist on produicing false declaration about my familly tree can I accuse him of diffamation
Thank you for your question. We are sorry you are having to deal with such an issue. When you find something inappropriate, you can report it to FamilySearch. The above link will outline what is considered abuse, and suggestions how to handle it.
Within the article you have linked above, the link within it to contact FamilySearch Support is "dead", so in practice the article doesn't help much.
"Under common law and according to the definition of this defamation, deceased individuals cannot be defamed. Defamation is defined as an act or statement that damages one’s reputation. The dead do not have reputations to damage. The memory of a deceased person can be damaged, but this is not addressed under the tort of defamation. Survivors or descendants of the dead have no legal claim on behalf of a deceased relative’s good name, nor can they collect on behalf of their own interests relative to that person’s reputation." https://www.minclaw.com/legal-resource-center/what-is-defamation/can-dead-people-defamed/
Just write up what you know about your family, support it with sources, and attach it as a PDF Document in Memories.
Also, analyze carefully the sources and documentation this other person gives. Maybe you will find that the information is correct and your family just never wanted to talk about it.