These are some general things that you should consider:
Your Moodle site security settings are available by navigating to Administration | Site administration | Security.
The IP Blocker page allows you to configure an Allowed IP list, a Blocked IP list, and tells you which list to process first. This is useful if you want to limit your Moodle site access to block access from IPs, which are sending you disruptive traffic.
The Site policies page allows you to configure a great many security settings, ...