Is it forbidden for a young man to drink from a cup of a girl who is not his mahram, or to eat from her spoon and plate, especially if he intentionally aims for the place of her saliva? Is drinking her saliva forbidden or makruh (disliked)? And is there any religious evidence for that?

Human saliva is pure, and what remains after a person drinks from a vessel (su'r) is also pure. Therefore, there is no need to wash a vessel that another person has used.

As for a young man intentionally drinking from the spot where a girl drank, in order to come into contact with her saliva, this act is forbidden in Islamic law. This is because it implies suspicion and a bad intention, and it could lead to what is unlawful. It falls under the principle of "blocking the means" (sadd al-dhara'i'), which is the prevention of anything that could lead to something forbidden. An example of this is the prohibition by Ibn 'Umar, may Allah be pleased with both of them, against sitting in a woman's place until it cools down.