How do you safely disable a plugin from the command line?

Hi,

I installed Plugin Manager on a 1.8 site without checking it out properly (it seems to be written for 1.7) and now my plugins page has disappeared to be replaced by the screen below. Hence, I can't work out how to deactivate Plugin Manager from the GUI.

I tried just removing Plugin Manager's folder from the mod directory, but that seems to break the admin panel.

How can I safely deactivate this plugin and get my plugins page back?

Any help gratefully received.

Tim