As a owner or admin, you can delete users. Head to your team settings > Members > Trash icon.
What about data?
All the notes created by this user in public or private channels will remain in your Slite.
All the notes created by this user in "my private channel" will be send as a backup link by email to this person. The link will download a zip files.
How to delete a user?
The backup email will also notify the user her/his account has been deleted.