Click on the selected transcription to open it, and then click on the button at the top labeled "Info" at the top.
Towards the top left of the info window you will notice in bold - Current Status, and Change to. Use the drop down arrow next to "Change To", and select "Invalidated".
Then click on the "Save and Close" button up top.
This will remove the transcription.
If you do not see the option for "Invalidated" then you do not have access to delete transcriptions. Please contact your transcription services provider for assistance.