It's a common problem. The only way I've seen where it's been solved is simply to have a system where people can add points (vote) to good ideas and then sort them by points rather than newest. With the exception that new topics will stay at the top for a few days to give them time to gather responses (effectively, they have hidden points that fade away over time that keep them near the top for a few days).
Then actually go in there and respond to the most popular ideas (and lock them once done).
Otherwise you might as well delete the board because it serves no purpose other than making it so that people who follow your boards rules are punished for it.