I'm using disqus comments system, but it keeps storing the comments in the wordpress database as well. Is there way to disable option to store comments in wordpress database because I don't see any use of it.
Did you already try disabling the sync from Disqus to WordPress?
(This is under plugin settings in the Disqus Dashboard view.)
FYI: The use of storing comments is to protect yourself against data loss if Disqus goes under, or if you want to migrate to a new plugin in the future.
Of course, you can manually sync with a setting as well, so barring an apocalyptic failure/shutdown of Disqus you're probably fine not storing the comments in your database.