Routing manually is not a problem.
But, if your answer to: not being able to create a routing rule, is to do it manually, then it doesn't speak well to the long term solution.
In complex PCBs it is a lot better to have a rule in place, even during manual routing, so that you don't have those frustrated moments weeks down the line, when your PCB arrives and you realize you should not have had vias in one area. Keeping track of each component's requirements would be a lot easier if you could create restrictions for that component beforehand.
In my case I have an MicroSD card slot that has a metal shield. No Vias are allowed in specific areas, but they are in other areas. This might seem like a simple solution to perform manually. But now I am expected to remember this for this specific MicroSD card slot, when ever I use it. Using Keep-Out Layer is not the answer either, since routing through the area is allowed.
@Andras: Sorry to say but your comment provides no benefit to the solution, since it neither suggest how to accomplish the task in Altium Designer, when we know it is possible in another software. You should know that rules still apply during Interactive Routing and I suggest you start trying them out for better results.