email verification step

I created a website that uses Joomla.
On the site is a Donation page using JoomDonation, which we like very much.

However, it is a religious institution's website, and unfortunately unscrupulous people use our donation form to attempt to see if (what I can only assume are stolen credit card information) to make small donations. As was further explained to me, if the card numbers work, they then try to buy large ticket items elsewhere.

So my thoughts on how to at least try stop the inconvenience of getting all these bogus donation email attempts and subsequent forms (I receive a copy of all submissions as the webmaster / volunteer ) was to include an email verification step.

How would I accomplish that?
Please keep in mind, I'm a middle aged, intermediate skilled designer on my good days.

Hi Steve,
It seems your Donation form is being spammed. I advise you to use Captcha feature to avoid the spam.
Thanks
Dam

Except the perpetrators are filling out the required fields. How would captcha stop that?
I did go ahead and add the captcha, so we'll see effect, if any it has.

Thanks,
SL

here's a typical one: (we got six different ones in a single day, and dozens over the past month)

Campaign The Anna & Leon Rudzin Holocaust Education Fund
First Name Barnypok
Last Name EoaXpqDGmlPMuxmfTx
Address PaYKPYDrooFpdl
City New York
Zip 38455
Country United States
State NY
Phone 31168491604
E-mail jfvynms4281rt@hotmail.com
Comment RR9D9G www.FyLitCl7Pf7ojQdDUOLQOuaxTXbj5iNG.com
Donation Type One Time
Amount $79.00
Transaction ID FK7EFKTJ
Payment method Offline Payment