There might be a quicker way someone else knows, but the way I've done it is to export the pricebook to excel. Find all the deactivated services you want to delete and delete them. Once deleted, save the file, and import it and click on deactivate existing price book. You might run into some errors if you had materials assigned to the services you deleted, in that case service titan will tell you what other lines you will need to delete as well.
If there is a quicker way then that I'll be happy to know too!
