You can manually un-install the agent from Add/Remove Programs or from the ServiceDesk Plus application. If you are unable to un-install the agent from the Control Panel then follow the steps below,
Click Workstations link from the Assets block.
From the Workstation list view page, click on Agent Details link. By default, the workstations for which the agent is not installed is listed.
Select the options either Agent with older version or Agent installed Workstations from the Filter drop down.
Select the check box beside the workstations to un-install the agent.
Click Un-install button. The agent is removed from the selected workstations.