Bogus donations through Jotform?

We have a donation form on our website: https://form.jotform.com/cordovacouncil/ccc-donations

Authorize.net said somebody is using our donation account to test credit card numbers.  They have actually found some legit numbers which have generated small donations to our account. 

We are getting dozens of these. I am not seeing submissions in our jotform account for these. 

Does this mean they have another means to access our authorize.net account. We are trying to resolve the issue and I want to confirm since we don't see these transactions through Jotform, that disabling the form will likely not fix the issue. Any suggestions would be appreciated. 

It was coming through a different donation form. Will adding a captcha resolve it?

You can use captcha or the unique submission feature in your form.

The unique submission would filter your form and won't let the same user with the same IP address or cookies to submit your form.

Guide:https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form

Please give it a try and let us know if the issue still remains.

Thank you.