You could use a solid region set to Solid ( or maybe No Solid) and set to a unique  net name. Note that if you use Solid Area set to Cutout, that creates a hole in the PCB. In either case you may need to delete the region after you have completed and checked the PCB.

@andyfierman but how to create a "band" (marked area, area between the two circles) of solid region? Seems to be impossible ... 

You want to be able to autoroute in the area inside the inner circle and in the area outside the outer circle but not in the marked annulus?

@andyfierman only in the inner circle and exclude the region marked ...

So what's happening outside the outer circle?

Try this? 1. Define a circular Board Outline of the diameter of the outer circle; 2. Draw a Circle of the inner diameter then right-click > Convert to Solid Region;  3. Then set it to No Solid and assign it a unique net name;  4. Copy it onto the bottom layer;  5. Then place your components inside it; 6. Run the Autorouter.

@andyfierman Thank you. I tried to do as you mentioned but without success. I will try again with a more simple layout and will post my result here in the next days.

@MiniMax, I'm still not sure that I have the right picture in my head of what you want to achieve. Any chance of uploading a sketch to clarify?