Looking for a "do not start program" signal

I am thinking of setting linuxcnc so that it will not start to run a program if my spindle is unclamped. Right now I have relay interlocks which prevent the spindle itself from turning if the spindle is unclamped, but there is no lock to prevent a program from running and moving the table. I am not sure I need this second interlock or not. I thought that if the spindle is unclamped and does not turn and a program runs, I could drive a not moving tool into a part, but then I realized that if the spindle is unclamped, it can not hold a tool. The more I think about it, being able to run a program without the spindle turning by simply unclamping and removing the tool, could be a good thing. Anyone have any thoughts? If I do want to prevent a program from running if the spindle is unclamped, does anyone know what signal I should tie into my spindle unclamped signal?

I think I would set it up so that if the spindle was commanded to run, and the tool isn't clamped, then an alarm would be set. I think this is how most of our tool changer machines operate (factory controls, not Linuxcnc conversions.) I think this would need to be this way to be able to perform an automated tool change during a run. The added bonus the logic isn't too difficult.
I'd use an and2 to trigger a message and drive motion.enable false, if motion.spindle-on and your unclamped signal are true at the same time.