How can I "clear" an entire website I'm selling the same website build in Craft to multiple clients and was wondering if it is possible to clear all user generated content of an install.

So delete all the entries, categories, assets, globals, ...
You could achieve this using a tool like schematic https://github.com/nerds-and-company/schematic

This allows you to generate all your fields and sections from a file without content. Combine this with a migration to seed dummy data (if you need that)

However, craft 3.1 will ship with a Project config which will do the same thing.