As I mentioned in this thread:
before patch 1.10 it was possible, but nowadays, you unfortunately can't do this with the only help of macros and addons.

The only possibillity i know is setting up the keybinding for Forward at your alts to another key then "W", and then just hit this key before you want to cast, and your alts will stop following your pally, and they are ready to cast.
So there is no solution which will autostop following, you have to do it yourself... (sad, I know )