Anyway to stop firmware update before rooting?

I just purchased a new S6 today from Best Buy. I was lucky to get a OE2 version, saw it on the box labe before I bought it. I would like to root and install a ROM but I want to wait until my 14 day return period is over in case I'm not satisfied and decide to do something else. The prompt to update pops up constantly. It's very annoying and I'm concerned I may accidently tap the update button. I'm coming from a S4. On the S4 I was able to use Root Explorer to rename a couple of system files to stop the request for updates. Since I'm putting off rooting until after 14 days that method is not possible. I cannot uncheck to automatically upgrade because when I go to system update I only get the "update ready to install" message. I tried booting to recovery and wiping system cache but that had no effect. Any suggestions? Thanks