What is the solution for a young man who stalks a girl electronically and intrudes upon her privacy, knowing that there was no real-life relationship between them, and the girl rejects any relationship with him?

What is published on social media pages is accessible to everyone, and there is no harm in anyone viewing it. If you mean your private matters, then look for programs that prevent hacking, and consult specialists. If you find no way, then do not write what you do not wish others to see. Do not accuse a specific person of tracking your secrets without evidence, for Islamic law has forbidden ill-suspicion. Allah Almighty said: "O YOU WHO HAVE BELIEVED, AVOID MUCH [NEGATIVE] ASSUMPTION. INDEED SOME ASSUMPTION IS SINFUL" (Al-Hujurat:12). The default for a Muslim is innocence until proven otherwise.