It should not supposed to be like that and we’ll definitely take a look at that in the future. But for now just delete the plugin via FTP from your site (wp-content/plugins/wp-math-captcha) or ask your hosting provider to do so. It will remove the math captcha field and authorization.
How to Get Support
After you register and login to the site, you may post all plugin support questions in the Support Forum.
If you need to provide private info, please create a ticket and then reply to it using Set as private reply option.