If you do not solely control the private key you do not own the bitcoin. This has been an adage in Bitcoin since the beginning. Unfortunately, in the case of online wallets, services, exchanges, or any other service, you do not solely control the private key and you cant protect it in any way. In order for the service to work they need to be able to move the bitcoin around. You deposit it to them and they control it on your behalf. If you are concerned about security move your bitcoin to a wallet you control, either desktop, mobile, hardware, or paper.