End-users cannot clear class or apply class of block elements (i.e., <p>, <div>, <h1>, etc.,) when a class name already exists.
The functionality will be available in Q3 2015. Once this release is live, you will be able to find further details about the update and instructions how to use it in this help article: http://docs.telerik.com/devtools/aspnet-ajax/controls/editor/functionality/toolbars/dropdowns/css-styles