Given how fast this grows, I would think that finding ways to push it back would be a high priority. Would it be as simple as flamethrowers and napalm, or would you have to nuke it to "disinfect" an area? A particular frequency of truly intense and focused sound causes break-down/disintegration, possibly. Maybe nothing at all works, but I feel that it would have been tested and noted in any case.
Also, does it go underground? Is it converting the soil and bedrock to some alien equivalent, or does the effect limit itself to on or near the surface? If it does spread down as well as out, maybe it isn't nearly as well contained as we think, huh?