I don't know that banning these sorts of networks helps *anything*.
1) It can be construed as a suppression of free speech.
2) It adds more fuel to the fire for those on the fence over the issue; if they feel that maybe these networks aren't supporting terror, they may wind up feeling that the US is simply stamping out voices of dissent instead of actual threats.
3) If they ban al-Jazeera, how am I supposed to do language training? Seriously, though, if you feel they are the enemy, shouldn't we have the capacity to "know our enemy"?