[ubuntu] how to hide paid apps from software-center?

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http://ubuntuforums.org – hello, since i have no ability at the moment buy applications, i want to hide "for purchase" apps from the software center. i did some digging online about it and so far found no solution.. - software-sources have multiverse entry, but that will hide non open source apps that distributed for free - in the software-center, under "All Software" button, there are several option. even if i choose to stay only with "provided by canonical" i loose the ability for filter by category, and its valid only for current session. (like here) - solution proposed here (Hardware)