With the professional brands we sell it is not usually possible to completely delete a section of footage from the timeline, it is possible to manually stop/start the recording to leave an unrecorded gap but not after the event. But you haven't been clear as to the brand/system you have and with some cheaper, consumer brands deleting footage manually to save storage may be possible. You would need to speak to the supplier/manufacturer the system came from to get their advice on whether it is possible or not.