Is a person considered sinful for concealing someone who has insulted religion and abused others, and what should he do to end this problem?

Cursing God and religion is disbelief in God (kufr billah), and there is no difference between one who is serious and one who is joking in this regard. Likewise, backbiting and cursing people are sins. These actions must be condemned to their perpetrator, and it is not permissible to sit with him if he does not stop them, based on the Almighty's saying: "And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they enter into another conversation. Indeed, in that case, you would be like them." It is also not permissible to use magic for revenge, because it is disbelief. Concealing the perpetrator of these actions is not permissible except out of fear of harm, and in this case, one should seek responsible authorities to deter him; otherwise, it is sufficient for you to avoid him.