Press Esc to close the schematic placement screen
Use the Create schematic tool to deselect or clear the current schematic
Right-click with the schematic and quill or schematic tool to remove the active schematic
Delete the schematic file from the schematics folder if you want it removed entirely
Unload or disable the schematicannon if it is currently using the schematic
Reopen the schematic menu and choose no schematic or a different one