Can i block IP ranges from VPN connection attempts?

If you have a hardware firewall in front of your VPN server (like I do) then it could be configured to block certain IP ranges from using the VPN ports. If not you might be able to do the same using the built-in software pf aka. pfctl firewall on the Mac.

The adaptive firewall should do that for you automatically, but if you're not satisfied with it, use a third-party app such as "IceFloor" to configure the packet filter. The ports to be blocked are UDP 500, 1701, and 4500 (for L2TP) and TCP 1723 (for PPTP).

The default configuration of the adaptive firewall doesn't actually work, though the documentation doesn't bother to mention that fact. Besides following those instructions, you have to edit the file /etc/af.plist. Change the value of the key "firewall_address" from the default "127.0.0.1" to the IP address of the interface on which the server listens.

So if I'm using DynamicDNS, the file would need to be edited whenever my ISP changes my IP? Or is it the local IP of the server?

Also, is there some way to test the firewall is on?

I don't really know much about Adaptive Firewall, but doesn't 127.0.0.1 just resolve to "local host"? I would've thought it would work the same here as with DNS? This is way outside my experience though.

It's true that the instructions don't work in El Capitan. What I did was to copy the file

/System/Library/LaunchDaemons/com.apple.pfctl.plist

to /Library/LaunchDaemons and edit the copy. Change the filename and the job label to something like "com.yourcompany.pfctl" to avoid confusion.