You can make it disappear by making the brushes at different elevations. It happens because the same one floor brush is on both sides of the wall. This is the sort of thing you have to take into consideration during the design process. Here's what I mean:
Notice how the tiled floor is on a different plane than the brown floor. This causes q3map2 to regard this surface as a separate lightmap. The issue you are having is because of how lightmaps are rendered. The floor surface you have has one lightmap on it, and the lightmap is scaled and linear filtered to prevent pixellation. Thus, the light bleeds. You have to either break up the floor surface or change the elevation to cause the lightmaps to be split.
You can also render your lightmaps at a higher resolution and they
maybe will not bleed. Try changing the floor brush into a func_group then using _lightmapscale to increase the lightmap size(try _lightmapscale 0.1 to make it 10 times as big as normal).
Also, keep in mind that some surfaces you will never see should be caulked. The floor surface under the wall will never ever been seen by players in a normal game, so clip the brush up and set the unseen surfaces to caulk. This can also help prevent lightmap bleeding.
You can leave lightmap bleeding in the game in some cases where it's useful for design. Light bleeding around doors is a nice touch because it actually tends to happen in real life.
