If these scheduled tasks are not running (because they are in the past) the only way I know to remove them is to us the attached SQL to remove all scheduled tasks. There is no way that I know of just to remove some of them.
I would recommend reviewing all the DB rules that use scheduled tasks and checking whether you can reduce the number of times a scheduled task is generated. for example by adding an additional condition to the rule which is not scheduled.
I think it means that you may have to check your DB rules, which schedules the actions. The DB rules which schedule actions to be executed in a future date/time, should (better) trigger on an attribute which is constant (Ex: Created date/current date/etc) and not on an attribute which can vary over a period of time (Ex: modified time stamp).