They did try! But the kids at 15 have less patience for trolls than they did at 13, and ignored the trolls’ messages until they gave up. Rose and Dave didn’t even bother looking at the messages. John looked at the first one he got (from Karkat), was convinced it was trying to sell him something and ignored the rest of the messages. Jade recognized all the trollhandles, got really annoyed because she honestly thought they had given up ages ago, and proceeded to ignore the messages and block each and every one of the trolls.
At the time, it started to look like trolling the kids was a really bad idea after all.