As an owner or admin, you may delete users. Head to your team settings > Members > click the Trash icon to the right of the member's info and role.
All the shared docs created by this user in public or private channels will remain in your Slite.
The private docs (in their private channel) will be sent via backup link, directly to their registered email (they will have 15 days to download their content.) The download link will provide a zip file containing their docs. After 15 days, this data will not be retrievable.
After you delete a member
The backup email will also notify the user that her/his account has been deleted.