Blaming the agency is no excuse - Pedder employ them, they are responsible.

If they did it to me I would do two things:
Notify them via Twitter (nice and public) that they (or the company they employ) has trespassed and caused damage to my property (nails in my fence? No thank you) and give then 24 hours to remove it.

If that doesn't happen, print off a nice big sign to put on THEIR sign saying the homeowner did not give permission for the Pedder sign. Not all publicity is good publicity, after all.