I'll be interested to see if there is a method, we have a client with >100 pages (easy to do with multiple scheduled daily reports), I've never tried to bulk remove old reports, but it would be good to know how if the time came.
Please note that currently there is no way to bulk delete call reports through the management console so you will need to do it manually. I would recommend posting this as a feature request in our ideas section of the forum https://www.3cx.com/community/forums/3cx-ideas/ . All posts are reviewed by their corresponding Departments and, if feasible and voted for by other users as well, may be implemented in an upcoming service pack or update.
If you however want to delete the already run reports you can do so by deleting the reports from the operating system.
For windows you can access C:\ProgramData\3CX\Instance1\Data\Http\Reports and delete the files. The reports will also be deleted from the management console.