You canno de activate/delete apex class from production directly. You need to use ANT Migration tool and create destructive changes.xm file which would be similar to package.xml and include the class name and deploy it to prod. The class wil be deleted from prod.
You cannot delete a component from change set. So what you can do is, create a project in eclipse for production. Open meta.xml file of your class in eclipse and change the status to InActive. Or you can delete the class directly from eclipse, it will delete the call from production also.
You canno de activate/delete apex class from production directly.
You need to use ANT Migration tool and create destructive changes.xm file which would be similar to package.xml and include the class name and deploy it to prod. The class wil be deleted from prod.
or you can check the recent post
http://boards.developerforce.com/t5/Apex-Code-Development/De-activate-Apex-Class-in-Production/td-p/610037