Just to re-iterate Buzz's point, its all about timing. The phrase "sh*t balls gets wickets" isn't heard without reason. So often people see a bad ball and as you rightly state, their eyes light up and look to dispatch said rubbish into next county losing all shape, timing and it seems ability.
Just like any other shot its about training the brain, practise hitting bad balls into particular areas (then you can adapt come game time depending on field placement) but when you start, begin by hitting the balls at 25% power, then 50%, then 75% before eventually putting them away with the authority you would hope to in a game without losing shape or timing - if you do, go back to the start and do not collect £200. This will develop another shot(s) to your repertoire, dispatching a rank bad ball is not about the blade travelling aimlessly at the speed of light but timing the ball with control as you would with say a wide half volley or the pull that you said you play well against seamers.
Just because its a toilet ball does not mean you have to play a sh*t shot! Hope this helps. If it doesnt - then i would suggest a complete overhaul of new batting kit!