If they come to my house I am going to politely ask to take a picture of the friendly nice person or persons, then after I have a photo to give police if any vandalism occurs I plan to sign the petition with F*** Y** as big as I can write.
First, your idea of taking their picture is a good one!
Second, I suggest you DO NOT sign their petition in any way. To these people, any signature, no matter how wrong, is counted as "another American supporting the President's policies".
Lastly, do not throw hate at them, even if they use it against you. We are better than they are and should not stoop to their level.
Remember, they would LOVE to incite an incident that could be twisted against us!
Though tempting, I would not deface their ‘property’ in anyway. Anything like that, in writing, could be distorted as some kind of threat. I wouldn’t touch their papers, their clipboard...but I love the idea of using my cell phone to make a photographic record of the ‘canvasser.’ A photo of their vehicle/license could also be useful should any subsequent vandalism occur. Just about every cell phone is a camera these days...put it to use!