I'm not sure about what kind of account you have, but I'll suggest to try this:
If you're administrator, you can go to the Site admin page - configuration > common site settings > Session types
(https://help.webex.com/en-us/WBX566/What-is-the-Site-Administration-URL)
- Look for the session type marked as "Default for new users"
- Go to settings for the marked as "default for new users". You will see many options there to enable or disable.
- Once you disable the "video" option, the users assigned with session type "default for new users" will not be able to use Video.
Here you can find detailed info about session types and how to edit them.
https://help.webex.com/en-us/acyd33/Manage-Session-Types-in-Webex-Site-Administration
Hope it helps!