Another user login in my erpnext and registered as system manager

i have a problem, someone that i did not invite, sign and log in in my erpnext system, and then he logged in with system manager role, how can i setting this system for my company email only?

@Evankhellim you can disable signing up

oke thanks

Like @bahaou suggested you can disable the new user signing up.

Or assign appropriate role for new user by configuring ‘Default Role at Time of Signup’ in Portal Settings.