Smite them with withering humour, laugh at them, make fun of them. lampoon them. However sad a funeral may be imagine if the mourners turned to those people then pointed and laughed at them, laughed long and loud.
Take every opportunity to laugh at them, make jokes about them,draw cartoons about them, turn them into a joke.
Remember Charlie Chaplin in the 'Great Dictator'. The point is to show they have no power over us, that they are insignificant. Of all things they would find the worst would be to be reduced to a joke and dismissed as such.
It would be hard in the face of such obnoxious people to find humour but people could do it, they would be have to be careful not to libel or slander them but a simple ' oh those Phelps people' followed by gales of laughter would do it. The trick is to appear not to take them seriously.