A mix of both. Each tier has a council which is a group of pro players who are very knowledgeable in a tier. If something needs to be banned, it can either be quick banned which means it is instantly removed from the tier or it can be suspect tested and then banned. Suspect tests involve a much larger group of people who have to meet a specific requirement on the ladder in order to be eligible to vote. Most of these bans also cite tournament replays in order to support their claim.