How can I remove all existing content in Capacities and start again?

The easiest solution is to delete any spaces with content in.

First, create the new empty space that you would like to use going forward.

Second, open the space you would like to delete. Open its settings, go to 'Space Settings' and click delete. You will be taken back to the homepage, where you can continue to delete any other old, unwanted spaces, or you can just start using your new space.