First, you aren't allowed to post URLs on this forum. So it will be changed soon by a mod, please read the TOS (the links at the bottom of the page) you were kindly asked to read before you signed up, it'll save you and the mods a lot of time in the future.
Second, 1.5% is VERY HIGH for YPN. I see usually a 0.2%-0.4% when the site generally gets a 4%+ CTR on Adsense. So 1.5% with YPN is extremely high.
There isn't really much more you can do without enraging your visitors.