You can add/edit the CustomUI.xml and make
<ribbon startFromScratch="true">
and then put things back
Here's a more complete example
<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui">
<!-- Set startFromScratch to true to hide the Ribbon and QAT-->
<ribbon startFromScratch="true">
<!-- startFromScratch="true" hides all of the Ribbon tabs and it hide the QAT. -->
<!-- It not hides the Contextual tabs on the ribbon, for example the -->
<!-- Format tab that you see when you select a picture on your worksheet. -->
<!-- So if you want to hide them you must use the RibbonX below: -->
<contextualTabs>
<tabSet idMso="TabSetSmartArtTools" visible="false" />
<tabSet idMso="TabSetChartTools" visible="false" />
<tabSet idMso="TabSetDrawingTools" visible="false" />
<tabSet idMso="TabSetPictureTools" visible="false" />
<tabSet idMso="TabSetPivotTableTools" visible="false" />
<tabSet idMso="TabSetHeaderAndFooterTools" visible="false" />
<tabSet idMso="TabSetTableToolsExcel" visible="false" />
<tabSet idMso="TabSetPivotChartTools" visible="false" />
<tabSet idMso="TabSetInkTools" visible="false" />
</contextualTabs>
<!-- You can add xml here to create your own custom tab on the ribbon-->
</ribbon>
</customUI>