I read somewhere that sperm contains the essential nutrients(incl protein), so does excessive ejaculation(masturbation) deplete these nutrients?

Easily replaced: Your spit and or other body secretions also contain protein and are replenished with normal feeding activity. Masterbation is a normal part of human sexuality. If it becomes excessive it can draw you away from social activities where you could develop more as a person. That would be more important than the trivial amount of nutrients in ejaculate.