You can't remove threads that you're 'on' we're afraid. You can remove threads you're watching though:
On the mobile site (which you're probably on on the iPad), visit the 'I'm watching' page, either by tapping the blue button at the bottom of the page or using the drop-down chevron menu at the top of the page. This is the direct link to the 'I'm Watching' page
From that page, tap the thread options (the ellipsis - '...' ). Unwatch is one of options.
On the desktop site it's the same principle - navigate to the 'I'm Watching' page via the blue shortcut menu at the very bottom of each Talk thread, or use the link above.