Well, using adblock is only going to work locally (you won't see your flag, but everyone else will), so if you don't want anyone to see your flag then you will have to ask WGT to do something.
But if you want to do the adblock thing you'll need to install it into your browser...
AdBlock
...then, once you have it installed...
- Click on the AdBlock icon in your browser toolbar.
- In the pop up window, click on the 3 dot menu in the top right corner.
- Click on hide something on this page.
- Click on your flag. (You might have to move the adblock hiding wizard if it's in the way).
- If your flag has gone, click looks good in the hiding wizard.
- Done!
EDIT : It hides everyones USA flag. But that could possibly be changed by modifying the filter.
EDIT : You're welcome. ; )