Sorry to hear about that. I've taken a look at your Stumble History and can definitely see the issue you are describing. However, we noticed that dumpaday has a feature that allows you to Stumble through its site and it looks like you may have inadvertently clicked that button. I have included a screenshot below.
It looks like you did the right thing by thumbing down the latest dumpaday Stumble you got, but you will probably get more because dumpaday has a very large number of pages in the Humor Interest.
You can go a step further and block the entire dumpaday site from showing up in your Stumbles by using the down arrow button to the right of the thumb down button and in the drop down menu that opens there and selecting "Block ".