Thoughts can be unlinked from the toolbar section of TheBrain. Keep in mind, any Thoughts that are unlinked from all other Thoughts will be considered an "orphan" Thought and can only be found via Search or while linking to an existing Thought.
Activate any Thought linked to the Thought you wish to unlink.
Tap the icon displayed in the bottom toolbar.
Tap the name of the Thought you wish to unlink.