How to remove a ticket with Real Work?

Hello, I'm stuck trying to remove some tickets which were imported from a spreadsheet with actual time.
Attached is a screenshot showing the problem.
I've tried setting the Assign Work (Dispatch Work) to 0, deleting the values, setting it from the ticket page - i've tried every combination of values (or blanks) to remove these but nothing seems to work.
Is there anywhere i can simply rip the values out of a DB table or another page i should be looking at for removing Real Work from an item?
Thanks

Additional: The same error message appears when i try and assign a time entry (ticket) to a task (activity) although the work hours values haven't changed.

Thank you, that worked perfectly!

After editing a few rows manually it seems to have unlocked something else as i'm no longer having a problem setting the Actual Work to 0 hours from the UI.
Hopefully i won't come across this problem again but it's nice to know there's an easy fix if it does happen.