RCW 29A.84.040 makes it a misdemeanor to remove or deface lawfully placed political advertising including yard signs or billboards without authorization. Check with your city attorney to see if your city has adopted this statute. If so, a violation can be charged in municipal court. If not, it will have to be referred to the county prosecutor.
For a bit more background, a local government cannot prohibit the placement of political signs in the areas between the street and sidewalk (or in the unpaved section of the right-of-way where there is no sidewalk), commonly referred to as the “parking strip.” As a general rule, the public right-of-way, which includes parking strips, is only an easement and the underlying property belongs to the abutting property owner. As such, only the property owner or the tenant of the property owner may determine what, if any, political signs are placed in the parking strip. In contrast, a jurisdiction can prohibit temporary signs in the untraveled area of a right-of-way that does not involve parking strips, such as in boulevard medians or in the middle of roundabouts. For more information, see this blog post, Regulating Non-Commercial Temporary Signs During Election Season, and our Sign Regulation webpage.