To remove one or several units, check them in the list and click on remove button in toolbar or select menu Network Units > Delete Checked Units (Ctrl + D). Confirm delete.
The units and all their collected information will then be removed from the database and the agents will be uninstalled.
Note that if a computer which is offline is removed, the agent will not be uninstalled. It also means that when the agent is started, the unit will be newly registered with the same unit-ID.