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Two ways:

First, in Dashboard, click the little "+" at the bottom left, and, when it rolls up and turns into an 'X', click the "Manage Widgets..." button next to it, then disable the ones you don't want.

Second, you can go to either Library->Widgets or [your account]->Library->Widgets (depending on where each is stored) and delete them directly.
 
when you're in Dashboard, click on the large "+" in the bottom-left corner. then click on "Manage Widgets", which appears next to the rotated "+". simply click on the round red symbol to move the widget to the trash

Edit: beaten to it:)
 
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