I really don't find it useful and piwik is been charging for me from a long time. Can anyone tell me how to discontinue my account with piwik.
I doubt that you are being charged by the Open Source Project.
Either you have bought an premium plugin in the marketplace: https://plugins.piwik.org/premium
Or you are using the innoCraft Cloud. Then you'll need to contact them directly.
Or you are customer of Piwik Pro, then you should look at this FAQ entry as this company is completly independent of the piwik project.