I think there are reasons not to destroy the fall.
1) The colony was a colony to UFB. So there was always some soldiers who were present in the colony and with whom the inhabitants of the colony was of no match in fight (Like Lori and the cybrogs). We have seen how much the dwellers there fear them. That is why terrorist groups of special persons were formed to fight the UFB soldiers.
2) @Oliver_c s logic is also quite true. Within 17 minutes it was hard to destroy a thing like The fall. I mean common it isn't a toy.
3) Another reason might be, we have seen that the troops can go to deadliest places like where Matthias lived by not using the fall. that means only destroying the fall cant stop them.
4) I also think, if they had destroyed the fall, this could be more dangerous to themselves as they were in every way dependent on UFB. They could not survive on themselves.