Guys, please update to the new 2.2.5. You'll find:
- user IP address is sent to Authorize.net for all attempts. So if someone is attempting fraud, you can easily pinpoint the IP to block.
- user IP is stored in DT Donate for all successful donations.
- added confirmation page to help reduce the opportunities for running scripts for fraudulent charges.
- CB plugin is also updated to work with newer versions of CB.
As stated by others here, tightening up Authorize.net security settings and using their Fraud package will help too as you can't stop someone from sitting down at their computer and manually filling out your form if they want to.