https://www.avvo.com/attorney-answers/53548-wi-jay-nixon-1529181/answers.html

Answer by:    Jay K. Nixon, Licensed for 40 years, Avvo Rating: 10 

Criminal Defense Attorney with offices in Janesville & Kenosha, WI 

While the owner could complain to authorities about your removing the bumper sticker as "criminal damage to property," I would be surprised if many prosecutors' offices would have time to do much with this police referral due to all the riot related, major arson and criminal damage cases which they are now prosecuting these days. Rather than risking it, however, why not just tell him that you plan to remove it if he doesn't? Either way, if it is a bad as you say, somebody else might very well remove it for you, or do something worse. Nonetheless, the law technically protects both his property rights, and his freedom of speech; so, acting without arguable permission would be illegal.