While mapvoting is going on, the text- and voicechat should be automatically muted completely. This is to prevent people from shouting out the map they want, which can actually affect map voting. This is not good, as people should vote for the map THEY want to play, not the guy who shouted a map out wants. As soon as the map is selected, both chats are unmuted again.
So, yeah, nothing more to say: Simply mute text- and voicechat while the players are voting for a map, so that players actually vote on their own instead of listening to other players.
(Because I see something like "DEATH BOWL!!!!!" and everybody votes for it, frequently.)