I already took an eye on this web site but didn't find the answer.
It is possible to avoid vias in particular areas by using "keep out" area or pour polygon, but you avoid copper tracks to. And i don't know how to tell Altium not using vias.
Place (pour) a hatched polygon on an unused mid layer, then use Tools Convert to free Primitives so the polygon is then free no-net tracks, the auto router will then not place vias in that area.