Occasionally, their motive is to shock or repel, as with gothic imagery or swastikas. If you get lemons, make lemonade. It is much better to pick up symbols that have obvious currency with students than to reject them as aesthetically, artistically or socially unacceptable. Putting down their symbols either throws cold water on the students or adds fuel to their fire. One can predict with almost psychic certainty when certain clich symbols will crop up.
Hearts are a perennial favorite at almost all grade levels.
Rainbows show up very early in elementary school, along with cute little mushrooms that pop up like, well, mushrooms. Puff lettering is a mainstay in upper elementary and middle school. The shock schlock is de rigeour in middle school, and sometimes, into high school. When hearts abound it is time for a unit on how hearts have been used throughout the history of art. Dig out all the paintings and sculptures that contain hearts you can find, show them, discuss them and get the students to talk about them.