To delete your workspace, go into the "Settings" section in the bottom left corner, then scroll down to the bottom of the page and click the "Remove workspace" button. Only the creator of the workspace can delete the workspace.
To delete your user account, delete all your available workspaces. After deleting the last workspace you will be redirected to "Add Workspace" page where you will be able to remove your account permanently by clicking "Delete my account" link.
Guides - How to archive and delete user
Guides - How to remove a workspace