As @Ankur_Khare said, the anonymous should not be able to create or in any way modify nodes in the JCR. If that is currently possible on your instance, then someone either screwed up massively or you're the victim of a serious hack!
However, if you are (for some reason) in a situation where the anonymous user (aka: visitors to your AEM website) can use the API to modify nodes, you should head over to /useradmin an remove any non-READ access!
Regarding the admin user, @Andrew_Khoury is right to say that this is a special user meant for (you guessed it) instance administration. This is the user you should give to your system admin or infrastructure department. If you are worried about developers using the admin user, you can change the password like so and give the password to the responsible party in your organisation (or preferably let them change the password).