As DarthViper suggested you’ll want to adjust the lightmaps resolution and/or the UV mapping.
Since you’ve tried upping the lightmap resolution and are seeing no change then you would need to start getting a good lightmap UV in place. 512 for a wall is a bit high when you can get away with lower with a good lightmap UV.
I’m pretty sure that the UV map is the culprit here. That map is a total mess judging from that screenshot and there seem to be several faces on top of each other. You should correct that in whatever 3d program you’re using.
From judging the screenshots, I can say that you not given enough spaces between UV island, you must have enough spaces between islands for the lightmap padding. If you are using 3ds max; make sure that you adjust your UVs by pixel snapping. Keep your UVs oraganized as much as possible, weird rotated angles might give some artificats.