To remove people from a review cycle navigate to 'Admin > Review Cycles'. Here you will see all the reviews created so far. 

From here select which review you'd like to edit. You'll then see the following screen:

To edit the review click the 'Edit Cycle' button as highlighted above.
From here click the 'x' located next to the person you want to remove from the cycle as highlighted below:
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The person will then be deleted. When you're finished click 'Save Changes as shown below. Alternatively to cancel changes, click 'Cancel'.
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Note:
Your changes won't take effect unless you click 'Save'.

Learn more about 'Review Cycles' here.