hmm if there's a texture for teamglow for all units (maybe, maybe not, cause most units have other glow positons, blademaster's sword is glowing too)
however, just use a alpha channel on it, open photoshop for example, where you can watch the chanels, and then just add a new channel, it's named Alpha 1 or something like that.. now erase and edit it like you want it to.. black = invisible // white = visible (if you turn of the view of other channels, if not, all parts, that are red are invisible)
So just create an alpha channel, I think it automatically makes everything invisible, so save it, convert it back to blp (was jpg or tga before, to edit it) and then import it back into the map, change paths to original ones.. and it should work..