How I deny an App that I already allowed access to my keychain?

I had recently downloaded the latest version of the Opera Browser app form Opera's website for web development purposes and website testing. I had the Appstore version but it has not been updated to the latest version. So I decided to delete the Appstore version and get the Opera's website one. The current version is the build: 15.0.1147.132... Yesterday I was messing and configuring the new version and at some point, OS X promt me a warning that Opera app was trying to access my keychain credentials and it gave me the options to allow or deny. I chose "allow"... I couldn't sleep last night because I was thinking that I did not did anything to make Opera to ask for my Keychain credentials and spent most of the night reseaching on OS X the option to deny that granted access that I gave to Opera app. If some one knows how to disable the access of an app that has been granted to access the OS X's keychain app, please provide me with the steps to achive this. Thank you😕