I though a pawn flt was released for that, or was it just for extra functions?
"Pawn flt"? All servers use the same client.
I've seen that a lot of people are under the impression that pawn is the client, this is not the case. Pawn is in no way related to the client, it is all to do with the server.
To make your server private you'll either have to use the working mIRC server, bad idea if you ask me, or just make sure that your firewall blocks the VCMPServer so that it won't announce to the master list. Some routers allow for NAT which automatically opens up servers to the outside world without the need for opening ports. However I don't think VC:MP Server uses that feature so just make sure your ports aren't open.
Your computer will have an Internal and External IP even when ports aren't forwarded, this is why people are joining. Just close off your ports and everything will be fine.