How do I stop a rolling chair from rolling on a warped floor?
There are three general approaches that will fix the problem: 


Make the floor level.  It isn't clear what you mean by "warped", but if the floor doesn't have any structural problems and you're talking about localized dents rather than general sloping, you could add a rigid laminate layer on top (like Pergo).  If the floor is dented, a plastic chair mat will quickly conform, so the chair will just roll on another layer.  
Lock the wheels.  Replace two of the wheels with locking wheels (not very practical in use), or use wheel chocks as ArchonOSX suggests (might become an inconvenience).  
Greatly increase the friction of what the chair rolls on.  Use a dense rug, as isherwood describes, or a very soft mat.  But this approach will also make it harder to move the chair when you're on it (and if you put a plastic chair mat on top to make it easier to roll, you're back where you started).