There is a KA for removing the decomissioned hosts.
Removing the entities from agent list will prevent them from being populate going forward.
You can dismiss them via the UI or via SQL (with the assistance and review of technical support.
Article Number 000027594
In our TrueSight Capacity Optimization (TSCO) environment we have a large number of systems that we would like to delete from the environment. Identifying and dismissing the systems via the TSCO UI is difficult. Is there SQL that can be run against the TSCO database itself to dismiss the entities? If so, what is the SQL that can be run to dismiss entities from the environment?