It is easy if you create the following macro on all characters and map it to a key.
Then use the "keypress-send to all - tool" of your choice like HKN, ISBOXER, Keyclone ...
look up the round-robin keypress function in that tool :
isboxer: steps
hkn: Toggle
Keyclone: ??? (dunno)
and you are ready
Code:/stopcasting /cast Thunderstorm
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