Hello.

Can anybody tell me what value needs to be altered to mark an object as not being allowed to have the CASt tool used on it, please?

I've been able to remove all of the CAStable channels but you can still attempt to use CASt on it in game which results in the game telling you the chosen colour scheme is not valid (as there are no channels to change) and to select a different preset, of which there are none.
The only way to exit CASt then is to select a different object so that the cancel button becomes usable.

In some resource (I don't know which) there is a 0 or 1 meaning enable or disable channel, per channel. But I think that leaves it *behaving* as if it is CASTable, without it actually taking any colours. I know that in a non-CASTable object, there are no TXTC resources. Maybe you could experimentally remove the references to the TXTCs which are in the MODL and MLODs

Also, in the OBJD and the Preset XML there are blocks for materials and patterns that you could try removing or editing. A non-CASTable object uses the overlay for its texture, and does not use the mask or multiplier.