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.
How can I merge two accounts?
It is not possible to merge accounts. You will need to export your space(s) from the account you wish to close and manually import it/them into the space you are keeping active.
What is the storage limit for my account?
There are no storage limits for notes, object, and spaces. On the free plan there is a storage limit for media uploads. You can read more about it here.