Nav avoid is ment to do that. But for this i wouldnt recommend it (mentioned later in the post)
There are multiple reasons why they still walk over it. The most common one is that it simply doesnt touch a full nav area. when using nav_edit 1 you can see that some nav areas go over that specific area. Even if its just 8 units it wont cover the avoid.
Also, normal bots although they dont have a visible tag still have the common tag. But those are only for small bots.
The other one is that the bot MUST pass the area to reach you in which case it is able to walk over it, although in most cases they will just stand at the edge (bat scouts and busters can occasionaly go over them). Since once they are inside such area they will ignore its avoid setting.
However, in general its better to actualy avoid that setting for anything except the bomber. Forcing bots in thin paths will simply make them inefficient in tracking down players. Especialy roaming bots (the ones without a bomb/objective) will usualy take those areas to attack.
The only thing in this case you could do is using a nav prefer on the central path to guide them to use those paths rather than the sides, this still allows free movement when they want to.
