Debug UI
You can a single crash report from Backtrace UI Debug view by clicking on the three dot icon to the right:
After selecting this you should get the following dropdown menu:
Morgue
You can also delete objects via the command-line by using the morgue command-line tool https://github.com/backtrace-labs/backtrace-morgue#delete .
For example, if you wish to delete errors with IDs 10a1, 10a2, and 10a3 from the project "projectname": morgue delete projectname 10a1 10a2 10a3