You can use our data maintenance feature to assist with that. Data maintenance allows you to set a retention period for your completed instances in Forms. You can set this at the global level for all processes, or you can set this for specific processes in process options. We offer three levels of clean-up: 1) delete all uploaded attachments to the instance, 2) delete instance variable data, 3) delete the entire instance. This allows you to set a retention period in days for how long you want to keep the instances after they are completed. 

If you want to delete all instances of a process, you can set data maintenance to delete everything that's over a day old. This deletion is permanent

If you have a bunch of test instances, you could set data maintenance to 0 days to clear them out, then turn it off or set it to something much higher for real instances.