What you need to do is modify the respective ToolsFile.xml or ListToolsFile.xml (for forms scenarios), which are located in /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/4.5.x.x__1f131a624888eeed/RadControls/Editor folder.
First remove <tool name="FormatStripper" />
line and then add <tool name="StripAll" />
I hope this helps.
the Telerik team