Handling the OnClientPasteHtml event of the editor cannot be used properly, because the logic cannot rely on the command name to further interact with pasted content.

For the time being you can use the following script to workaround the issue:

<telerik:RadEditor runat="server" ID="RadEditor1">
</telerik:RadEditor>

<script type="text/javascript">
	Telerik.Web.UI.Editor.CommandList.PastePlainTextWithTable = function (commandName, editor, args) {
		if (editor.get_lockFormatting && !editor.get_lockFormatting())
			return;

		if ($telerik.isIE) {
			var restorePoint = editor.createRestorePoint(),
				utils = Telerik.Web.UI.Editor.Utils,
				dirtyText = utils.getClipboardAsHtml(editor),
				cleanedText = utils.cleanPastedContent(dirtyText,
				editor.get_stripFormattingOptions(),
				editor.get_localization()["askwordcleaning"], false);

			restorePoint.select();
			editor.pasteHtml(cleanedText, args.get_commandName());
		}
	};
</script>