Yeah, I got that too. It asks you to fill in the last four of your credit card and the amount you donated (and zip code). It shows the receipt, but if you want to cancel, you have to email them.
You have a remarkably good attitude about being robbed. Can I have your address? I promise I only burgle once. 
Any interesting article about Actblue:
**
How ActBlue Is Trying To Turn Small Donations Into A Blue Wave
The liberal money-gathering platform is remaking fundraising for the left.
**
I’ve donated via ActBlue and then been peppered with email requests for more money.
Every damn thing is a Chicken Little event that requires my immediate help which I can do by sending them more money.
I am sympathetic to the cause but thoroughly annoyed by incessant pleas for more money. It has the opposite effect on me.
That said they never scammed me and when I unsubbed from their shit their emails stopped. They never charged me for things I did not authorize.
Don’t worry he’s probably drinking an ice cold margarita with your money as we speak. 
I strongly recommend having a separate email address that you use for when you buy stuff online or donate money online. That way, all the junk email traffic that inevitably results from such purchases and donations goes to that separate address, and doesn’t clutter up the address you use for emails with your friends and family.
At any rate, the firehose of campaign-related emails that need your immediate help will taper off real fast next week. Hang in there. 
No they shouldn’t (be able to take any electoral victories for granted). They should all resign, don sackcloth and ashes for the rest of their lives, and disband the Party. And EVERY SINGLE eligible voter in the country should vote against them until they do.
Does it say it is an ongoing thing? Did you ever actually get a charge on your card?
Because one thing that would fit your description would be that they incorrectly sent a thank you for a recurring donation they didn’t take. It would explain why you couldn’t cancel if the site thought it was a one-time donation.
I donate sporadically via ActBlue and haven’t had any problems. Although I think that the ongoing donation button used to be the default, and you had to remember to click on the onetime only button instead. Once I forgot, and I noticed in my confirmation email that it was recurring. I can’t remember specifically now how I handled it, but I know it took no time and all and required no phone calls to switch it to a onetime donation.