This is not possible right now. You can see how the Graph component is calling the plotly.js methods here: https://github.com/plotly/dash-core-components/blob/b7f776f4aa15be3dae160a4df7b011a4eaa7f425/src/components/Graph.react.js#L66-L78
The rest of the modebar config options are outlined in this forum post: Is it possible to hide the floating toolbar?