there are a number of ways to protect your RTMP outbound stream. Please see
this article for some suggestions. For example, you could apply a simple
username/password protection to the stream. More advanced topics can be found
I guess it depends on how they're getting your Wowza RTMP URL. You could apply
outbound security using IP whitelists and blacklists, if you know the VLC player's IP address or if it's on a different subnet or network which you could then block.