The opened window is the html-viewer and not the cccc-plugin.
Normally it has a frame with at least a close-button, but this might be removed by your window-manager.
You can remove it by resetting the current perspective to the default. In this case you will lose the settings for toolbars etc and have to recreate them.
"View -> Perspectives -> Delete current"
You can also try to drag the window (maybe it works, even if it seems to have no border) and dock it somewhere in C::B.
You should now get a frame with a close-button to close it.
Could you specify your OS, version and used window-,anager, so I can try to reproduce it and see if it is a wxWidgets, Code::Blocks or window-manager issue.