Hi IC-DaMan, :welcome to armaholic
In your server.cfg file you should add the following option
voteThreshold=2;
By setting it to 2 it means 200% has to agree with the vote.
This means people can still vote but there is no way any vote would make a valid result.
Also have a look at this topic:
http://www.armaholic.com/forums.php?m=posts&p=7681#7681