You need to go to https://steemconnect.com/apps/authorized and there you can simply click revoke for any app you don't want to authorize anymore. Then a transaction gets broadcasted to the blockchain, that means, that you will need to have some RC for doing that.
The easiest way is to log into steemconnect.com at their official site. Once you have logged on and been verified by email you can change your settings manually.
There will be list of accounts that you have authorized and you can just untick it as such. very easy and straight forward. this will change the authorization for that particular site.
Check out following Steemit post to learn how to revoke dapps via SteemConnect https://steemit.com/steemit/@lolicorn/how-to-revoke-posting-permissions-for-steem-apps-via-steemconnect