Concerning "нельзя": when you specify a person, who "cannot" do something, it may only mean prohibition. "Туда нельзя заходить" also implies that one "shouldn't" come in. However, "Туда нельзя зайти" does indeed mean impossibility.

If you really need to specify a person, and still imply impossibility rather than prohibition, use verb "мочь":"Ты не можешь туда войти". You can even use perfective to say it in the future "Ты не сможешь туда войти" ("You won't be able to come in there").

Note that English uses "you" extensively where Russian doesn't. For example, it is normal to say things like "You can't just steal other's work". In Russian you would rather say an impersonal sentence with "нельзя": "Нельзя просто красть чужую работу". So don't bother with following grammar too closely: it is perfectly normal to use the most usual way to render the idea.