If you delete a conversation from your conversation list in Fleep, then you delete its history only for yourself. Any other members of the conversation will still be able to see the conversation and its content.
If you are a Fleep for Business admin and delete the conversation with the "Delete permanently" option from the Admin panel, then the conversation is deleted for all members, along with its message history.
Read more: How to delete a conversation? and How to permanently delete a conversation for all members?