If I understand you correctly you want the unclosed div tag to stay unclosed. This is, however, invalid HTML while the purpose of the RadEditor control is to provide valid XHTML content. There are a lot of content filters
in the control that take care of similar problems.
Even if you disable the content filters we have the browser's rich-text editing engine will step in and add the closing </div> tag. This is a browser behavior, which we cannot influence, and what I can suggest is that you validate the content before saving it into the database. You can see on the following demo how to use ASP.NET validators with the RadEditor: Editor - ASP.NET Validators Support
. You can use a custom validator and modify the content as desired or at least notify the user that the content is not desired.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed