First step is i would set Kentico's Url Extensions to be empty (Settings - URLs and SEO - Friendly URL extensions should be empty)
Then also on the same settings page, set "Redirect page aliases to main URL:" to true
Lastly, make sure that your web.config has the runAllManagedModulesForAllRequests="true" on the modules element so the above will work.
That should do the trick, you may still get 301 redirect notices on your SEO scans if you have hard-set links that you'll need to adjust, Kentico generated links should no longer have aspx though.