[SalesForce] Disabling email notifications for Community user in salesforce

Easiest option might be to just update all the contacts to have fake addresses using data loader, or if the number of contacts would be within governor limits you could run a small script in the execute anonymous section of the developer console:

list<Contact> toUpdate = new list<Contact>();

for(Contact c : [Select Id, Email from Contact])
{
  c.Email += '.fake';
  toUpdate.add(c);

  if(toUpdate.size() == 200)
  {
    update toUpdate;
    toUpdate.clear();
  }
}

if(toUpdate.size() > 0)
{
  update toUpdate;
}

Obivously you could add some more smarts to this if need be, for instance limiting to 20,000 records and only grabbing contacts where the email doens't contain .fake and run it a few times. If you're cutting sandboxes on a regular basis then maybe you could adapt this code to be a class implementing batchable and throw together a quick page to execute the batch when needed, deploy that to your production org and you'll always have it to hand right after creating/refreshing a sandbox!

Have you tried the settings on the NetworkMember object? For Communities, this seems to take precedence. For a similar issue, I poked it via the developer console with this query:

SELECT Id, PreferencesDisableAllFeedsEmail FROM NetworkMember WHERE MemberId = ''