Cyburbia is a friendly big tent, where we share our experiences and thoughts about urban planning practice, the built environment, planning adjacent topics, and anything else that comes to mind. No ads, no spam, and it's free. It's easy to join!
The solution to your problem is simple: Amend your subdivision regs so only trophy homes on giant lots can be built in the future, then raise your property taxes and adopt huge impact fees for future development. Pretty soon those little pests will flee your jurisdiction and your worries will be gone. You can use your new additional tax increment to get a good transit system going so the people who make your city function can be bused in from far away.
Sorry for the sarcasm ....On a serious note, look for ways your office can help remedy the situation. Start with adding good design guidelines to your code, allowing small density bonuses that offer incentives for older properties to upgrade, contact HUD or your state for info on housing/streetscaping grants, set up community meetings to get people involved. I heard about a woman in Tampa, Florida (a "layperson" who proved more effective than her local planning department) who organized neighborhood barbecues while residents cleaned up their streets and homes with grant $ she acquired through her non-profit.
Oh, how true--if the neighbors, the governing body, and local bureaucrats let it happen. Neighbor peer pressure is the most effective deterrent to decline, especially if there is is good maintenance code in place. I have found the 'middle class' areas are more concerned about property appearences than others.