How to prevent the torch from moving up?

Not sure what you want???
Moving torch up is basic plasma stuff, so i do not get it why would you want it down during changing material?
To break it easier?

The torch height is controlled using external offsets:
linuxcnc.org/docs/devel/html/motion/external-offsets.html

It is not a material change that causes the torch to rise, it is part of a sequence of events stated by a spindle stop (M5 $0) or an error/abort during a cut.

You could possibly prevent it with a bit of hal work on the axis.z.eoffset-enable pin. You would need to be very careful is this would cause repercussions later if the Z axis is not where it is expected to be.

Bl484 wrote:

Another question:
The safety height parameter controls the amount of rising of the torch.
But I cannot set this parameter to zero!
If I could have set it to zero, my problem would have been solved.
Do you have any suggestions?

It that will work for you then I will have a look tomorrow. 

A quick an dirty method to test whether this may work for you is to change the value via HAL while QtPlasmaC is loaded.
You cannot use zero as there is an inbuilt fallback which sets it to maximum Z height if zero is detected.
I am assuming a very low value will be good enough to test with.

Can you try the commands in a postgui Hal file.

If it is worthwhile I could change the code so that you could set it to zero from the parameters.