Captchas are mainly to prevent your form from spammers and bots. However, you can enable/disable the security check (captcha) from your form options.
To enable or disable the Captcha option, navigate to the Settings tab of your form either after creating/editing your form or by clicking the Settings option from your Forms page.
On the Basic section (the default open tab), toggle the button below the Do you want CAPTCHA for this form? heading on/off. Switching it on (green) enables the Captcha, while switching it off (grey) removes the Captcha from your form.
Your options are saved automatically. You can click the View Form button to see the changes made.