Just for the record, I am not saying all of this is Google's fault. I can accept the initial mistake was completely user caused.
I was just teasing, but still the title of this thread and the content of your OP...
But anyway..
What I DO have a problem with is the fact that Google+ continued to upload almost 2GB worth of data via 4G after (a) the option was changed to WiFI only AND even worse (b) the backup option was completely turned off. I still don't think that behavior is acceptable. If you do, that's wonderful for your wireless carrier I suppose.
Now this is something I actually experienced as well, in a different way. I have two Google accounts on my phone, and sometimes I like to switch around which account my phone's photos are uploaded to. One time in a freshly installed phone after I added the accounts I mistakenly checked the 'update' box on the wrong account, so I immediately turned it off, flip to the other account, and set the update there.
Then I waited for the photos to show up in the second account, but nothing comes out. It turns out that since the update process already started on the wrong account, instead of switching to the correct account it just keeps uploading to the first account until it's finished (no more new photos to upload). This wasn't a big deal to me so I let it, but for the future I know to be careful with this type of thing.
So you are correct, a Google+ autoupdate setting change won't retroactively affect a sync process that's already started. I'm guessing though you can kill the process by force-stopping the right application (either G+ itself or a separate sync process, not sure), or probably an easier way just restart the phone.