Currently there is no way to mass delete clients or individual clients. Theres no way to easily implement this as clients have so many tables linked to them especially for billing and accounting tickets etc this could cause major problems throughout the app if a client was actually deleted causing mass breakage all throughout the app.
I think the best solution here is to implement bulk archive of clients. Currently clients have no bulk actions but would be useful to implement bulk archive, tags, and edit industry, maybe we can squeeze this in before the next release.