Abstract: An online system organizes content items based on assignments of the content items to categories within a hierarchical taxonomy. The online system tracks the progress of each online system user towards frequency caps applied to the categories. A user who reaches a frequency cap applied to a category is included in an exclusion group associated with the category, making them ineligible to be presented with content items associated with the category. Upon identifying an opportunity to present content to a viewing user, the online system determines whether the user is included in an exclusion group associated with a category. If not, the online system may present a content item associated with the category to the user, update the user's progress towards a frequency cap applied to the category, and determine whether to include the viewing user in the exclusion group based on the updated progress and the frequency cap.