I think existing laws covering "disorderly conduct" should be applied to Phelps and his ilk. In Arizona, it's covered by section 13-2904-A
A. A person commits disorderly conduct if, with intent to disturb the peace or quiet of a neighborhood, family or person, or with knowledge of doing so, such person:
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3. Uses abusive or offensive language or gestures to any person present in a manner likely to provoke immediate physical retaliation by such person; or
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B. ... Disorderly conduct under subsection A, paragraph 1, 2, 3, 4 or 5 is a class 1 misdemeanor.
If he was arrested and tried, I doubt any jury would acquit him!