The Leadspace panel can be removed by deleting the Visualforce section in your SFDC page layout.
You can do this by going to "Setup" => "Leads"/"Contacts"/"Accounts" => "Page Layouts" => then click "Edit" on the page layouts that contain the Leadspace panel.
Once you're in the page layout, scroll down to the Leadspace section, and click the "-" icon in the top right hand corner of the Section. This will remove the panel and the section.
Once you have removed Leadspace from all Lead, Contact, and Account page layouts, you can then uninstall the Leadspace Managed Packge. To do this, please follow these steps:
1) Go to your Salesforce Setup
2) Search "Installed Packages" in the Quick Search
3) Next to the "Leadspace Managed Package", click "Uninstall"
4) Authorize the package removal process
5) If you encounter any errors while uninstalling, it's most likely due to Leadspace components still present on one of your page layouts