There is no uninstaller. Quit lite.computer, open your Applications folder, and drag lite.computer to the Trash. That removes the app.
A Showspace is a folder you chose, holding ordinary HTML files. Removing the app does not touch it. Every page, every comment and the theme you picked stay exactly where they are, and any web browser can still open them.
Open Settings from the account button at the bottom of the sidebar and choose Reset this Mac. Press it again to confirm. Your Showspaces are not touched.
Install lite.computer again and paste the same license key from your purchase email.
Lost the email? Ask for it again at support@lite.computer and we will resend it.
Still stuck? Write to support@lite.computer.