There is no configuration option to disable alliances in PHost. Alliances are always allowed. However, there is an option to control players' access to the alliance configuration commands: CPEnableAllies. If that switch is off, players cannot form or break alliances.
If you wish to configure a game without alliances, you would disable CPEnableAllies, and then create a universe where no players are allied. This is the default in major universe creation programs.
Disabling CPEnableAllies also allows to configure games with fixed, predefined teams: create your universe as usual, and then set up the team alliances. Players will not be able to break these alliances. See Executing Commands from the Host-Side for an example.
