You have the option to delete some of your targets (including their scan results) and add new ones instead. Or you can upgrade to a higher plan, which allows you to add more targets and keep your scan history.
From the /targets page you can delete one or several targets by selecting the checkbox right next to it and clicking Delete. Be careful though, deleting a target will also delete all it’s associated scans.
You can also move one or several targets to another workspace by clicking the Move button.