McMansions can be opposed in a variety of ways but one poster suggests the way to go is to be an undesirable neighbor:

Paint your house bright pink. Put several cars on cinderblocks in the front yard. Have 20 people move in with you. Stop cutting the grass. Park junk cars on the street in front of the vacant house. Blast loud music 24/7. Tie up a pit bull in your front yard. Get someone with a huge gut to hang out in your front yard without a shirt while drinking beer. All of these together may work, but you’ll probably make yourself miserable in the process.

These actions may or may not be possible given local laws and neighborhood regulations but they all have a similar goal: drive down property values so that possible McMansion no longer looks financially appealing. As numerous people will tell you, who wants to have the nicest house on the street, particularly compared to your immediate neighbors? If McMansions are about wealthier people upgrading their property regardless of their surroundings, then such actions could undercut their financial basis.