The deleted and flagged posts section on the user summary page can only be seen by staff users. Regular users will not see your deleted or flagged post count.
Clicking the link should display the deleted posts. I’m not sure why this would fail. There is no way to remove this through the Discourse UI. The only way I can see to remove deleted post count for a user would be to completely delete the post by removing it directly from the database. Directly removing a post from the database is not recommended.
Only staff users on your site will see the deleted post count, so possibly this isn’t something you need to be concerned about.