Iodine does a good job.....blueberry juice also
Poison Ivy ;-) Just rub poison ivy all over the sign.
Tanglefoot: single-strand barbed wire strung between stakes in the ground in a meshwork pattern. The wire should be at about ankle height. A barrier designed to make it difficult to cross the obstructed area by foot. Usually placed around permanent defensive positions.(sharpen the tops of the stakes for effect)