I think you got it wrong. The 'Enabled OnBeat' triggers every beat: at that point the frame counter is reset. So unless there are 500 or 750 frames between 2 beats, your particles will never disappear.
but the effect list is enabled, and i cant turn it off, it keeps popping back to disabled
And if there are too many frames, i have no difficulty changing it.
I've got it !
When the number of set frames isn't reached before the next beat, AVS automatically sets that list to 'enabled'
Just read the comment on the added test