App Permission and Force stop
SMS and MMS are general, system wide messages available through the Android API. Messages by any third-party app are independent from that and cannot be retrieved without reading the original database files containing those messages. (Which basically means it involves a rooted device/root permissions)
If you force close an app it is shut down and doesn't run. It does not remove any permissions what also means that another app may launch that app and that app may read your messages then.


Apps that don't run are not doing anything but the permissions persist. Using an application that is able to manage those permissions can revoke those permissions permanently.