Well, your second question sorta answers your first. I mean, normally, you couldn't Go Aggro with a weapon that takes three days to work, and if you tried, people would do all the things you suggest in response. You could use your access to water as leverage for the Manipulate move, but Go Aggro? Nah. So that's what the Move does.
Now, as for what people can do about it during the three days...well, there are a couple of options. First, I think you can always choose to rescind your ban, since even an hour or day of deprivation can qualify as d-harm, so you can give them their water back if they convince you to do so before the three days are up...but they'd need to convince you.
See, I'm pretty sure by the very nature of having the move, you have the sort of moral (or other) authority to make that kind of threat in such a way that people can't just kill you or take the water to prevent it from happening. Heck, maybe if they try the water dries up (at least temporarily)...The Source is pretty overtly supernatural, after all. They can presumably Go Aggro on you or Manipulate you to get you to rescind their death sentence...but if you choose to accept the consequences of not going along with such moves, they all die.
As a codicil, I think this pretty much is only gonna happen with PCs, and only to a community if there is a PC Hardholder. I mean, NPCs are very rarely willing to die in agony over days for almost any reason, certainly not whole communities of them! Heck, I'd expect a PC Hardholder who forced the Waterbearer's hand on that move in regards to a whole community to be overthrown and be given to the Waterbearer in hopes he turns the tap back on in the vast majority of cases.
On a 7-9, as usual, they are forced to do one of the listed actions, or suffer the consequences.