There is currently no mechanism to stop rendering from inside a SWF - but it would be possible to implement. Although - I currently have no time spare to look at this in the next few weeks - but would be happy to add that as a feature when I do (which would depend on your code being AS3 / and being able to run in local context).
For now - If the length of your SWF is dynamic you will need to find a way to tell SWFRenderer externally the length of the movie and use
```
stopMode=0 // Stops after time ellapsed
stopTime=5 // Number of seconds to capture
```
But I presume you already dismissed that as a solution. Maybe if you have a local script it could also load that same variable and calculate the length of the movie ?
For now - (this could be a very long shot - if it works) - maybe throw and error from inside your SWF - that might cause the rendering to stop ? - but I would not count on that being a solution (have not tried it - it is just a total guess)