You can delete errors from within the Backtrace UI by using the trash can icon next to the crash when looking at the group view:
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