Can you do it?

[Job]

Can it be arranged so crossing a linedef spurs a never-ending flow of spawning demons to occur, which can be only shut off by a switch?

[Ichor]

It shouldn't be a problem. Try something like this:

For the first trigger:

script 1 (void)
{
    Thing_Spawn(tid, type of demon, angle);
    delay(whatever length of time);
    restart;
}

For the second trigger:

script 2 (void)
{
    ACS_Terminate(1, 0);
}

[fraggle]

Not in normal doom. Although I guess you might be able to pull some kind of kung fu with boss spitters, perhaps.