in every gametype there is a setting caled "MapLimits=0". Set it to the ammount of maps, which have to be played, until a map can be voted again. The button "restart" won't be affected by this. To avoid restaring a map (which means, voting the actual map again) you must set "bNoRestart=true" in the [UT2Vote51.UT2VoteConfigs] section, but this will prevent map-restarting for all gametypes.
BTW ProAsm, i guess, we could explain the settings of the gametype-switches a bit more, or did i miss them? Ini-setings are well documented, just the gametype-settings are a bit "poor"