Once you've fixed an issue, we won't hide it until another scan is run. You can start a new scan in your account. If the issue is fixed, it will no longer show in your dashboard.
If you notice recurring false positives in your diagnostics results (e.g. slow loading pages that are PDFs, links incorrectly appearing as broken), simply check the box next to the false positive and click the Hide Checked button at the bottom of the screen.
While we'll keep detecting the issue, it will no longer appear in your dashboard. You can always see hidden issues by clicking Show Hidden at the bottom of the diagnostic issues page.