If you don’t mind restarting Little Snitch every 3 hours, you can use the demo version beyond 30 days.
“Without a license key, Little Snitch runs in demo mode, which provides the same protection and functionality as the full version. The demo runs for three hours, and it can be restarted as often as you like.”https://www.obdev.at/products/littlesnitch/download.html
If you are willing to consider a cheap alternative, rather than free, then Vallum for $15 would work - https://vallumfirewall.com/.
I own their pro pack which is Vallum combined with Murus. If you ever also want to consider a packet-level firewall tool, the Murus is a fabulous application for that. That would allow you to easily, for example, block traffic to a particular IP address.