The Octane plugin should be deactivated automatically when you close Houdini or any other non-GUI tool like hython, but be aware that if Houdini crashes without a proper exit the license could be locked to this machine. In this case, it is enough to launch Houdini again and render something to unlock it after exiting. There is also an hscript command "Octane_releaseLicense" that can be used from the Houdini textport or even from hython to release the license.
Hope it helps,
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini