Other than the client offering the flyer for resale, what circumstances would you begin legal action? If the client changes some copy for example?
I don’t get into that situation because I know intellectual property law and would never allow the rights to graphics not to be assigned to the client. They may not ask for it, but I ensure they get it. Most people have no clue how IP law works and do not use lawyers, but I find it is only ethical to ensure they understand their rights and that there is the specific assignment of rights to them.
One issue with trying to strong arm clients on artwork is that they also have trademark law on their side. It is simply best not to haggle in that area of law. Know the law, ethically engage the client with the law, and don’t ever try to strong arm someone over IP issues.
However, I did have software that I wrote that a client did not want to pay for because they wanted a million dollar software package for peanuts, and I had to exert copyrights in that case to get paid. That was a client I had otherwise refuse to engage but political concerns cause me to accept them against all better judgment.