To disable a digital slider with a button on a chelsea switch you will need to create two scenes.
One scene will be the scene for the PC scene for the digital slider.
The other scene will be for the chelsea switch. This scene will have the same relays that have PC programmed to them in the digital slider scene programmed as either 0% or 100% (whatever they want the light level at when the slider is deactivated). You will then program that scene to any button on the chelsea switch as ON MODE.
When the chelsea switches scene is activated it will disable the digital slider scene since two scenes can not have all the same relays programmed to them.