Can I block an IP address?

This is the only suggestion Apple provides.

Block connections to your Mac with a firewall - Apple Support

As BobTheFisherman says, the default Router setting for unsolicited incoming connection requests is to DISCARD them at the Router.

You are ALREADY protected (by Your Router) from any connections, unless YOU initiated them. If you intend to host web services and deliberately open Ports on your Router, that is far beyond the casual use under discussion here.

There is no need to enable the Firewall on your Mac unless/until you are no longer behind a Router you control AND TRUST. At the coffee shop? in an Airport? Maybe you would like to turn on your Mac's firewall if you do not trust the agency administering the Router to do that already.

If you are behind a router on a private network, outside devices can not access a device on your private network without you setting up port forwarding in your router.