You can't generally cancel a running coroutine easily. 
You can of course do 

======
rename runningCoro ""
======

But then: 
   * the coroutine doesn't know it was cancelled(*), no error is raised inside it, it just ceases to exist
   * if the coroutine has scheduled itself as a callback like in e.g. 
======
::chan event $chan readable [list [info coroutine]]
======
just like say [coroutine::util gets] does, then that event will produce an error. 
Unfortunately I found no way to handle coroutine cancellation without resorting to callbacks. The good news is one can do
======
proc cleanup {args} {...}
proc coro {} {
    ...
    trace add command [info coroutine] delete [list cleanup $arg1 $arg2]
    ...
}
======
Which will allow it to e.g. unsubscribe from fileevent. Quite awkward: another proc has to be created and it also has too accept mostly args that [trace] passes to it in addition to any useful args we may pass from inside the coro.