Force-Quitting Dashboard Widgets

Before opening the Dashboard, locate the file associated with that widget in either /Library/Widgets/ or ~/Library/Widgets/, where ~ is the folder in the Finder's sidebar with the house icon, and throw it away.

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..." The problematic widget is actually the widget management widget"...

That particular widget is located inside the Dock.app bundle itself so it might be hard to locate, but removing it wouldn't be recommended.

Since the "Widgets" widget isn't a part of the default set, it shouldn't be present after deleting the "com.apple.dashboard.plist" file in the affected user's "~/Library/Preferences" folder. Since you mentioned that "none of the traditional ways [to "close" that widget] have worked" (btw. it would help if you stated what methods were actually tried), it could be that that file is corrupt. To make the change take effect, it will probably be necessary to restart the "Dock" eg. by quitting it using "Activity Monitor.app".

"Repair Disk Permissions" from Disk Utility may help you.
You can also create a new user account and check things there. This will tell if the problem is account specific or not.
Did you add any new fonts lately?

Mihalis.

You can also click on the widget in Dashboard then use hotkey Command-R to reload it.

The widgets provided by Apple are located in the /Library/Widgets/ folder of your hard disk.

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