You have to use a modeling program, like Milkshape 3D to edit them out.
I'll give you a simple tutorial, even though i think there's one or two in the tutorial section.
1) Open map in insolence and scroll to the SBSP tag.
2) Click on the blue plus sign, go to collision and export collision to .obj.
3) save it as Turf.obj to somewhere youll remember. (It's important that you add the .obj.)
4) In milkshape, go to file/import/wavefront OBJ and find your file, and open it.
5) Now in Milkshape, find the highest vertices, or whichever ones look like invisible walls. Select them. Do not select the edges of the map, because then you can fall through and if becomes annoying.
6) Now you have two choices,
A) Delete them, but this causes problems sometimes and will give you no collision points in certain areas of the map.
B) Move them, but remember that they'll still be there so if you move them under the map it MAY cause invisible walls within the map.
7) I'll go ahead and delete them because I think they'll make invisible walls all over the map if I move them down.
8 ) Export as a wavefront OBJ, in the same place you did with Entity, but make it have a different name so that if you screwed up you can still edit the original.
9) Go back into Entity, go to the SBSP tag you should already be in, go to the blue plus sign, BSP Collision, inject BSP collision mesh and find your new .obj.
10) It should have a pop up box say "test," say OK, sign the map and FTP.
I just tested the map, and it worked fine. No no collision spots on the map.
