No, you have to buy the hacker plan and make those private repls, so they can't see them or form them, If you want then feature without hacker plan then I recommend suggesting it on feedback. Or maybe using another code editor.

https://repl.it/feedback

No there is no way to disable people from forking your repl. The only way to do that is to buy hacker and have private repls so people can't see your repls and can not fork it.

No. Why are you asking for it?