The marking is not a core function of JSViz. It is something that is implemented as a helper. Certain situations require a different approach, such as Lassoo marking, or to disable it altogether.
To disable it, simply edit the JSViz.js file and remove the section that implements the mousedown, mousemove and mouseup handlers. This is all contained in the initMarking() function, so simply remove the call to initMarking() (line 293 in the current version) will do the trick.