I think the way you stop walls from being ubiquitous is reducing their value. They are a lot of work to build, but they are extremely useful to the point they are quite literally necessary. I propose they should be for a specific purpose - Slowing down potential invaders, rather than acting as impenetrable barriers (if you ram check.)
There are two solutions, one severe, one less severe.
The less severe is to make people pay decay upkeep for their walls even if they are on claims AND allow rams to be invulnerable during their dry period. Essentially, they provide 24 hours notice of when you will need to actively defend your walls.
The more severe is to allow walls to simply be scaled by a consumable hook & grapple, that is fairly resource expensive.
---------
The obvious effects of this will not only be that walls will become far less common.... but they will also not provide absolute protection for people that leave scents. Leaving a scent will actually risk your life, rather than be a minor annoyance.