How can I stop an account from automatically linking with Facebook?

Is it possible for a friend to use an old account of mine (which I let him use) and connect it with my facebook account, even though I already have another new account linked to that same facebook account?

I seem that when he uses my old account, it will still connect to my facebook. I told him not to link it but it still automatically connects and displaces my new account. How can I stop this from happening?

He has iOS, I have android.

You have 2 options:

• Disable linking from Facebook in Clash of Clans (Go to Settings, and you'll find it easily)
• Like @Kovah said, delete the app from Facebook (In the left menu > Applications)

As far as I know, it's the same process whether you're on Android or iOS.