That's because the parser for the kick command only grabs the first part of the person's name. This is why, by default, Frug has designed the chat to put an underscore "_" in place of the space of a person's name.
The kick command, as well as the private message command, parse the entered text and separate it by each space into a text array. The array is then parsed to find the command entered (the first element of the array), and the username submit (the 2nd element in the array).