If the same person keeps speaking up in open plenary discussions and/or speaks for long stretches, you can react by invoking the cooperation agreement or suggesting that a rule should be added to the agreement. The last round of discussion at the end of the workshop presents an opportunity to bring this up as a disruption. However, the best ways of off-setting dominance of the discussion are found in your working methods. You can use the circle discussion method, or the barometer, or you can split the participants up into small groups, etc. Not only this, but you should make room for more people to participate and use the options available to you to inspire or encourage them to speak.