Yosemite Server, adaptive firewall not stopping screensharingd attempts?
The only sure solution I know of for this is to configure VPN on the server then lock down port 5900 on the WAN interface. That will kill the attempts for screen sharing from the public side but allow it when you VPN into the server.

If the port is open on the public-facing WAN side, you can be sure that the port-scanners are going to find it and attempt getting in.