You can 'retract' a vote. Retracting refunds your SUP, and also puts a penalty of sorts on the task completer. However, be certain that the person failed to complete. I've been retracted when I in fact completed the task. For me, I haven't gotten too bogged down in going after incomplete tasks; rather, I subtract the cost of my investment for each task, against the gains made on my posts. I'm invariably far ahead.