PROP USERSPERIP
The USERSPERIP property allows operators to control how many users should be allowed to join the channel per host address. This will let you decide how many people from a single IP address can use the channel at one time. As soon as the limit is reached the next person that tries to join will get an error telling them that they can't join the channel.
Privilege: Channel Operator
Attribute: Read/Write
Examples:
/prop #lobby usersperip 4