Kicks players that send 3 or more messages that are at least 16 characters long and at least half capital letters. A warning is sent after each message fitting these criteria as well.
Players with the 'basic_privs' privilege are exempt.
Do note that this is a pretty old mod and was originally a one-off thing for one specific server - as such, it's not very configurable and probably not up to the sort of standards that other such mods may be.