There is not an option to dismiss Accessibility issues in bulk for the entire account. You can dismiss issues in bulk with a CSS selector, but this is limited to a per site basis. You can view which issues have been dismissed as "Can't fix" or "False Positives" from the Accessibility > Activity page in the platform. Occurrences that are dismissed with a CSS selector will be visible in the Activity section. In the "Source" column, it will list the CSS selector, and show the affected pages that it applies to.
