Picketing funerals...First Amendment?

while they unfortunately have the right to pickett. they cant say certain things. this is the key to stopping them, the minute they start in with calling the deceased a homosexual then that is going over the line. because it is defamation of character. thats when they should be sued, but no one so far has done this. only law suits filed under the first ammendment have been done. but if every one sues for defamnation then they will stop after the first few law suits