Export your customized entity, extract customizations.xml and replace the the ribbon definition in the customized entity with an uncustomized ribbon definition. Replace everything between
<RibbonDiffXml> and </RibbonDiffXml>.
To get an uncustomized ribbon defintion, you can export the same entity from an uncustomized CRM organisation. The default ribbon definitions for Microsoft Dynamics CRM can also be found in the SDK. (If not a custom entity.)
Here are instructions to export, extract and import the ribbon xml.
http://msdn.microsoft.com/en-us/library/gg327952.aspx
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