If you use the built-in DNN rich text editor (FCKeditor), you can custromize its toolbar to show only those toolbar buttons that you want to provide to your users. Each security role can have its own set of buttons, and you can configure the editor for each module instance, all instances of a module or for the whole portal.

To customize the editor settings, edit an arbitray event and click on "Show custom editor options" below the editor. This opens a dialog window, where you can change the settings. For your purpose, you can select the "Basic" toolbar for your event editor role (in section "Editor toolbar view options"). The Basic toolbar has only buttons for bold and italic text and; I think, for lists. You can even create your own toolbars, but I have no experience with that. For more information, please visit the FCKeditor forum.

Hope that helps!