Are you referring to this app http://apps.splunk.com/app/1539/? If so, the "Domain/Host" drop down menus and the Overview dashboard use a saved search to populate the domain list. Therefore, if no new data is indexed for a hostname, it will automatically age out of the lists after one month.
sourcetype="rum" | stats count by u_host
earliest="-1m"
If you want to remove the data sooner that this and you have delete permissions in Splunk, you could execute this:
sourcetype="rum" u_host="www.example.com" | delete
Or modify the saved searches not to include the domain:
sourcetype="rum" u_host!="www.example.com" | stats count by u_host
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