Disabling 'text' option in add event form
We utilize WordPress’ wp_editor function to create the description editor. I took a quick look at the arguments for the editor at : http://codex.wordpress.org/Function_Reference/wp_editor. However I don’t see a quick option to disable the “Text” input.
The tribe-community-events.class.php file’s formContentEditor function is responsible for creation of the editor. I’d suggest searching within the wordpress.org forums to see if there are any details to modify the editor as you desire. If possible you should be able to modify the community events function to suit.
Sorry there is no quick way to disable this feature.