What can I do so noone can unlock the door from the outside

There is no way to disable this lockset from its intended purpose. "Privacy" aka..'Bed and Bath' locksets are designed with an emergency release feature so that in the event of.. a child locking himself in a room, someone slips in the tub, etc... no key is required to gain access. It is a "Privacy" feature only. If you wish no one else to be able to have access to a room, then you are in need of a "Keyed Entry" which requires a Key to unlock. Any further questions please call our customer service staff at 1-877-4GATEHS (1-877-442-8347).