What do you say about someone who is given evidence for the prohibition of a certain thing, then says, "I am not convinced, and this thing is normal and causes no harm," and is there any advice for such people?

The ruling regarding a person not being convinced of the unlawfulness of something varies depending on the reason. If it is due to the lack of clear evidence for the prohibition, then there is no sin upon him, provided he is qualified to examine the evidence. However, if the apparent texts indicate prohibition and he rejects them out of caprice or arrogance, then he is sinful, as Allah Almighty says: {And whoever opposes the Messenger after guidance has become clear to him and follows other than the way of the believers – We will turn him to what he has turned to and admit him to Hell, and evil it is as a destination} (An-Nisa 4:115), and His saying: {But no, by your Lord, they will not believe until they make you judge concerning that which has occurred between them, then find no discomfort within themselves from what you have decided and submit in full submission} (An-Nisa 4:65).