By Jonathan Allen NEW YORK (Reuters) - New York police have begun handing out fliers to tourists in Times Square telling them that tipping the costumed superheroes and children's characters who pose for photographs is optional, and to call the 911 emergency number if they have complaints. The fliers are part of an aggressive crackdown after a string of arrests of people dressed as characters who have gotten into confrontations with tourists or police, including one dressed as Spider-Man charged last month with punching a police officer who intervened in a tipping dispute. "Tipping is optional," the fliers say in capital letters, printed in five languages in partnership with the Times Square Alliance, which promotes area businesses. "If you have any complaints, talk to a police officer or call 911." Police said they arrested three more characters on Saturday, the first day the fliers were handed out: men dressed as Spider-Man, Iron Man and Elmo.