Call the city. If that is public space, he can do this, and it is not actionable. The space before a lawn is usually public sidewalk. If it is against some city ordinance, they can cite him, but you can not. You can see if a local lawyer will send him a cease and desist letter, but he is not obligated to comply. Also, the lawyer will have to see if there is a legal basis to send the letter--if this is public area, then I'm not sure he/she will have one. As for a lawsuit, I don't see what the cause of action would be.
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I don't think there are any rules about putting property in front of another person's house since it is part of a public right of way. However, you should check your covenants to see if by doing so they are violating the nujsance rules. You can also call the city and see if there are any restrictions that aren't obvious. Your letter would reference what you found about any rule breaking or you would, as a last resort, indicate that they are causing a common law nuisance by interfering with your use and enjoyment of your property and ask them to cease and desist or you will request an affirmative injunction (but this gets expensive to do)
I will add only my suggestion that you avoid even the appearance of threatening him with criminal prosecution to coerce them into paying. Such an actual or apparent threat could get you into some serious trouble.