Well it is kind of Strange that the Box would have been CHecked... I am going to guess that it might have been a Used camera and the person that you bought it from set it up so they could connect to it after you installed it over P2P...
So if you don't want any outside access to the camera I would go and turn off the P2P access and make it so only your local network could access the camera..
Here is what to know...
Outside access to camera without Port Forwarding or any other Network Magic is over P2PI have all mine on but if your worried about someone making connection you can turn that off...
All of mine that I installed new and used have never had that box checked... With P2P on only need the SN of the unit to make a connection to camera from outside with that Login enabled now without it being checked it would need a user name and password...
So I would if it was mine and I wanted to take care of things.. I would do a Factory Reset at the camera, then log in over admin account it will ask for a new Password give a good password, Then go to the User area and make a new ADMIN account called what ever you want. Like say Frorose could be the name you use like your user name make a password and that would be what you connect with This way if someone was trying to be a jerk and lock out the camera admin account your Frorose admin account is still there..
In future when you buy IP cameras I would make sure 2 things one they are new and 2 the seller is owner of product becasue even if not new maybe a return Amcrest would Preform a Factory Restore or reset before shipping it to another person because in the past there was people that had footage uploaded to the Cloud of the owner before... First thing I do now NEW or USED camera is Preform a Factory Reset before I even start using it...