How is app using cellular data after selectively turned off?

To answer your question about whether there is another way to prevent cellular data usage on a per-app basis: no.

If you disallow cellular data to be used by a particular app, then it cannot (or rather, I should say should not, in case there actually is a bug in iOS) access your cellular data. It's important to note that the number you see below the app (256 MB in your case) shows the total amount of cellular data consumed by the app to date, since the last time you reset your cellular data statistics. It seems as if you previously allowed Solitaire to use cellular data (and it has cumulatively consumed 256 MB of data), and only recently you have turned it off. Perhaps you are merely misinterpreting the number underneath the app?

Now that you have turned off cellular data usage for Solitaire, do you notice the data consumption increase? If that's the case, then iOS has a serious bug. If not, then everything is working as it should be.