On the page you linked to, the featured image in the post appears to be controlled by the plugin, as it has the class tribe-events-event-image
If that is the case, you need to check the settings in the plugin to see if you can prevent the featured image from being shown in the post single page.
Are you doing this only for specific post, or is that something you are doing on all posts.
IF global, on the weaver side, the setting to use would be Main options > Post Specifics > Featured image > FI Location Single page > Header image replacement.