What movie has a sign for aliens to not enter?

The movie you're referring to is likely "Signs" (2002), directed by M. Night Shyamalan. In the film, a family discovers mysterious crop circles on their farm, which hint at an impending alien invasion. The sign for aliens not to enter is a symbolic representation of their attempts to protect themselves from the unknown threat. The tension builds as they confront the reality of the extraterrestrial presence.