With the 1.1 server patch there is a boot option:
This is from the server help file:
IN GAME SERVER WINDOW:
Clicking on the in game server window opens up a pop-up window that allows you to BOOT and BAN Players from the game.
Boot: Click on a player name to highlight it and press the BOOT button. This will remove that player from the game in progress. However, that player will be allowed to rejoin the game and future games you host using the PC Dedicated Server GUI.
Ban: Click on a player name to highlight it and press the BAN button. This will remove that player from the game in progress. That player will not be allowed to rejoin the game as well as future games you host using the PC Dedicated Server GUI. That players IP address is stored in Banned IPs.
Banned IPs: Displays a list of all IP addresses you have banned.
You can type an IP address in the text box and press the BAN button to prevent players from that IP address to join games you host.
You can type an IP address with a wildcard “*” in the text box to prevent players from an entire range of IP addresses from joining games you host. For example: 123.123.123.* blocks all IP addresses from 126.96.36.199 to 188.8.131.52.
You can type an IP address with a “:” plus a number to define the IP address with the port to be banned as well. For example: 184.108.40.206:80.
You can revoke a banned IP address at any time by simply clicking the IP address and pressing