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You will have to undo the checkout. There is a right click option available and I believe "undo checkout" will work. One of the right click options should allow checkin the Event Frames such that you can delete them.
It sounds like a bad configuration might be causing the backfilling queue (request list) to become stuck. The queue is persisted to disk to allow you to restart the service and continue the backfilling queued requests. In the past, I would follow the procedure describe here: https://pisquare.osisoft.com/message/59118
For more advice, it might be best to wait for an OSIsoft employee or give technical support a call.