To disable performance rewards for selected affiliates, first you have to activate the feature Performance rewards and Commission groups.
In Commission Groups you can add affiliates to the group to apply the commission group settings for them. Usually you are using commission groups to have ability to move affiliates into the particular groups based on their performance. But if you want to avoid applying the performance rule
to some affiliates and let them stay in the same group, you simply need to set the affiliate's status in the particular commission group to "fixed". Now, even the rule should be applied to this affiliate, he/she will stay in the same group.