I have do this, but I can´t rmember well. Try this:
Before routing right click over PCB and choice Options/Routing Layers, then choice only Bottom Layer ant then proceed with routing. I don´t remember well. I expect be usefull. Regards.
but enabling only bottom layer will cause whole board to be routed in bottom layer only. I want only certain component and region to be routed in bottom layer
I simply put a rectangle drawn with lines on top layer. So autorouter can never pass over it. After autorouting I remove this rectangle. This may be the shortest way.
The second way is appliying rules and layers for each component, net or area... This may take more time for small projects...