Unfortunately, Purging/Clearing DNS data is currently not possible within Umbrella. Logging is either enabled or it's not, but there's no way to purge data from the Dashboard after it has been collected.
You can choose to turn off logging completely, or disable Content logging, which shows Security-related information and Total Requests only. This is done on a policy-by-policy basis. If you would like the ability to clear logs as an Admin, please file a feature request to see if this can be added to the product here: https://support.umbrella.com/hc/en-us/articles/230671987-Setup-Guide-Customize-your-Policy