Billboards are not telephone calls.

Harassment by phone is direct, immediate and portends escalation; billboards, not so much.

However, California is prosecuting someone for posts on a Facebook page.  The posts are anti-Islamic; the page is an Islamic page.  The posts could be worse - I've seen worse directed at Catholics in my comments, at least.

Also, Facebook has a "block" function, which seems like a simple solution that doesn't involve the government picking and choosing what "annoying" comments get prosecuted.