These guys simply don't quit. They are either really persistent and don't mind earning their share from time to time with these things or everyone is lying when they say their demands are not met and they don't pay them.
I wonder what's really their motivation...
Every website or forum has to learn to counter these sorts of attacks and it is pointless paying them to stop because if you pay then they will either attack you again or someone else will when they know that you pay up.