In networking, you generally either have an authentication mechanism, or you don’t.

It sounds like you don’t have “control” (can install a vpn) on the client devices. This makes authentication difficult. We need some aspect of the client that the server can use to make a decision.

Without touching the client, there’s only really two details we can use - the source ip address of the client, and the port that they are connecting to.

If a client wants to connect to the default minecraft port, it could be a scanner - but if it’s non-default, then the probability of being a scanner is much lower.

A firewall to do geo-based ip blocking will also cut down significantly on noise.

After that, minecraft’s built in authentication is pretty good.

With all of the above, we would know that the connector is coming from an allowed location, knows to ask for your non-standard port number, and has a valid minecraft account - that sounds pretty good to me.

And if you’re running a cracked server, there are other assorted tricks to avoid bots. I ran an open-to-the-world, default port, no auth server for some time; and probably ran into a single robot. Thankfully I shut things down before log4j