You could have a macro on a specific toon, whether it is an interrupt or a crowd control.

#show
/stopcasting
/cast Whatever

If you're using IS Boxer/HKN, you could go with a toggle.
The key has two steps/states.
One step disables the normal action for a specific character and enables an alternate action.
The other reverts behavior to the default.
Depending on how your DPS is set up, you could have the interrupt/cc target the focus and when enabled prioritize over dps, to be used once every "10" seconds or whatever.



Let's say you're using IS Boxer.
Your default spam key is "1".
When you press "1", a bunch of things happen.

Step 1 (Press 1, or Odd presses)
- Do Mapped Key FTL Assist Me, Target: Current Window
--- Your slaves acquire the same target as your main.
- Do Mapped Key CC/Interrupt, Target: All Windows.
--- This key is deactivated by default, but can be toggled on, on a per character basis (ie a toggle per toon).
--- Default state is "off".
- Keystroke Action, Output Key: "1"

Step 2 (Press 2, or Even presses)
- Do Mapped Key: FTL Assist Me, Target: Current Window
--- Your slaves acquire the same target as your main.
- Do Mapped Key CC/Interrupt, Target: All Windows.
--- This key is deactivated by default, but can be toggled on, on a per character basis (ie a toggle per toon).
--- Default state is "off".
- Keystroke Action, Output Key: "2"

So when you spam "1", you get the slaves to assist the main, and alternate between "1" and "2" on subsequent presses of the key.



Then you have a round-robin key.
Each step has your other characters target the active window.
And has one of them set the target as their Focus.
If it is an npc, you can place a raid icon over the target too.

This sets your focus, for the interrupt/cc effect.



Define your CC/Interrupt mapped key.
- Keystroke Action, sends "3" to the Current Window.

In Warcraft drag your CC or Interrupt for the encounter onto the "3" key.
This will be a macro, with stopcasting and whatever effect, targeted at your Focus.

If you want, you could get more complex.
Like two steps.
1: Send "3" to the current window.
2: Do nothing, for 100 seconds.
x: reset to step 1, "12" seconds from the first press.



Create a toggle per toon.
So five keys in a five box set up.

Each has its own hotkey.
Two steps.
Step one enables the CC/Interrupt mapped key for the given toon.
Step two disables it for the toon.