Just to clarify, no we don't use actual balls!
During rush we had a rating/voting system, you rated someone from 1-10 on personality, scholarship etc. if you just didn't like someone you could give them a low score.
But if someone has done you or those close to you serious harm in some way - just for an example - you could give them a "no" vote. The no vote is VERY serious. You don't use it for shallow or petty reasons. It's saying "no matter what the circumstances, I could never call this person sister." It takes 1-3 of these votes to eliminate a person for membership.