Is there a way to stop a simulation?

As far as I know it is not something commonly needed. Perhaps you could revise your question or add a comment describing why you would like to do this.

If what you are trying to do is just end a simulation after a certain number of Monte Carlo steps that is easily done inside the main Python file using something like this:

cc3d=ElementCC3D("CompuCell3D")
potts=cc3d.ElementCC3D("Potts")
potts.ElementCC3D("Steps",{},20000)

One possible way to do what I think you are asking is to use the semaphores that the Player uses to pause or stop a simulation. To do this you would have to pass the simulation thread object to your steppable.

In the main Python script there is usually a line similar to:

sim,simthread = CompuCellSetup.getCoreSimulationObjects()

When you create your steppable create a parameter for the simthread object, something similar to:

def __init__(self,_simulator,_frequency=10,_simthread):
    ...
    self.simthread = _simthread
    ...

Then in the main Python script when you instantiate your steppable you would pass the simthread object to it:

mySteppable=MySteppabe(sim,simthread)

In your steppable when you would like to pause the simulation all you need to do is:

self.simthread.semPause.acquire()

To restart:

self.simthread.semPause.release()

To completely stop (end) your simulation:

self.simthread.setStopSimulation(True)

None of these functions (except of course for the ones setting the number of Monte Carlo steps) were meant to be used this way however, and you will certainly mess up the Player's button control during the course of your simulation.