Well I agree that you should be able to attack and possibly kill the alien blobs. It wouldn't make sense that they disappear from lack of nutirents because we can't really say for sure what blobs eat. Green blobs could eat anything even survive off of micro nutirents if it had too so being able to eventually kill them is not a bad idea. btw You can attack blobs I just don't know if its worth the effort though and I've never killed an Alien blob by just attacking it.
As for a soloution to how to make fire a more rounded item, its a toss up. Fire, if allowed access to a healthly supply of oxygen, will burn for a long time, but being able to attack fire? I don't know about that. theres too many varibles. To bad there are no sci-fi water spitters.

Oh and I agree that once used, fire and Alien blobs should be obstcals for everyone and everyone should be able to attack the green blobs and not just your opponets. Alien blobs get in the way of everyone after all. Space stations/post can be used by all right? well i'm done typing for now.