you must edit event plugin
edit event/controllers/base.php
find class EventAddForm extends Form
and the find
$desc = new WysiwygTextarea('desc');
and change
$btnSet = array(BOL_TextFormatService::WS_BTN_IMAGE, BOL_TextFormatService::WS_BTN_VIDEO, BOL_TextFormatService::WS_BTN_HTML);
$desc = new WysiwygTextarea('desc', $btnSet);