Preceeding it with an exclimation point would work best because that character is almost never used to start lines (unless the line is something like "!!!!!!!!!!!") and is commonly found in similar uses, such as in mIRC scripts. "!vote kick #" seems to be the best, and most simple possible format for such a feature.

I would also assume the !vote command would have other applications, such as "!vote map x". Because of this, AkuKitsune's suggestion of having the command be "kick vote #" isn't really consistant at all.

If you wanted to try something different, you could have the | character (or multiple | characters) be used in place of the !. That would make "commands" be shown in colour.

Have commands delivered through normal chat though. If for some reason a player tries to give a nonexistant command or makes a typo, don't make the program say "Invalid command" or something; the lack of a result would tell the player the command didn't work just as well as any error message could.

I assume that this feature would be integrated into that program of yours nimrod was showing off last AB. If this is the case, perhaps add an "!info" command or something which returns the server settings, such as time limit, cheat protection, idle kicking, the next map (or whether map voting is enabled), server admins (if that feature is in) and whatever else can be set with that program. Since that would probably take multiple lines, limiting !info to being used every 30 seconds or so would be a good idea to prevent spam.

If this program supports server admins, obviously have commands for kicking and banning, as well as commands to restart the level, go to the next level, set the next level, and a few other basic things.