Suing is very rare. Before that people send a email to the site owner simply stating that they are not complying with the license. Then they usually already feel the heat. Like if they had to change the domain name to something else then wigglypaint, then they cannot ride on the visibility wigglypaint already has . They would start with zero-visibility, which for low-effort clones, means many of them will get way less traction. For this approach it helps to have it explicitly stated in the license to quote it.