Scroll to bottom of screen and select View Mobile Site. Click in each thread you want to hide then click on arrow top right. Chose Hide This Discussion from the drop down. Once you have hidden the ones you want to hide, select View Desktop Site. It won't prevent the threads appearing in Trending as that isn't customisable but it will stop them appearing in your Active List.
Cariad I regularly hide threads as it clears the page so I have a clear view of the threads I am interested in. Some threads would otherwise just sit there after they have run their course.
Go to the upside down triangle at the top right (on the phone version). Click on it and options will appear. Hide is one so click on it and the thread should disappear