You can block an IP address from outside your network in one of two ways.
The first method is to use Filter Rules. This is located at Config > Network > Filter Rules. Select Source Address as a condition to block traffic originating from the IP address that you want to block. To block traffic destined to the IP address, use the Destination Address condition. Filter Rules block traffic at the network layer.
Another method you can use to block an IP address is to use the Firewall app. This method has the advantage of being policy-dependent: Filter Rules are applied to all policies, whereas Firewall will only block in the policy that it is installed in. Also, Firewall is a layer seven application, so it has additional blocking options when compared to using Filter Rules.