You'll need to build the game as normal. This will KEEP the hostgame button.
Now make two copies of your game folder and in one of them delete the image knxrb mentioned above. This will be your "Client version" which you'll distribute.
However, simply removing the button's image will not prevent them from adding their own image. If they know the workings of FPSC, they can easily alter the appropriate script to use a different image or make their own.
It's all in the script for the titlescreen. To truly prevent someone from using it, you'd have to package your game into an EXE that will, at a minimum, only temporarily extract the needed files for execution.
The one and only,
