http://source-docs.blesta.com/class-Clients.html
Looks like you need to use class Clients.
getByUserId( integer $user_id, boolean $get_settings = false )
Fetches a client with contact details of the primary account holder
Parameters
$user_id
integer
$user_id The ID of the user
Then you can delete by client id
#delete( integer $client_id )