The only workaround was clicking on the 3 dots on the right side and deleting them individually, or by selecting “Archive Disk Cache” under the “Store & Forward Engines” tab.
In theory, you could look at the REST API documentation or just open the browser console and see the request we're trying to send to delete multiple items from quarantine, and then duplicate that request yourself in an external tool.
It's possible that the backend is failing to handle this request, though, so it may simply not be possible.
If you're able to, could you either
report this into our support department officially, or
capture the browser console error(s) (and any errors in the gateway logs) and post them here as preformatted text?