Once you have more than 6 customs visuals the display window gets all messed up and you can no longer scroll down to the "Remove"/"Cancel" buttons.
So basically the field with the custom visuals expands until it reaches the end of the "window" but you are no longer able to see the buttons or scroll down to them. The scroll bar only works in the custom visuals field.
Of course you can still yout TAB or arrow key to navigate to the buttons.