I too am troubled by this. I am assuming that the settings when I bought my computer allowed for 'install important updates' and hence the reminders to update. The big problem I have with this, beyond not being able to control if I install 8.1 or not (I have since read the advice below about turning the option to update off) is the COST.
Windows say it is free to install. However, it has cost me close to $120.00 in broadband fees. This is unacceptable. I only realised Windows had decided to install Windows 8.1 when I discovered my data usage going through the roof. I have since managed to look at the data usage record and found 2824MB data used on 5 December and 2828MB data used on 17 December, among other large amounts. I thought the upgrade required about 3GB data so am at a loss as to where this amount of data has been used.
I know my usage is generally very low so having to pay this much for an upgrade which I didn't really need is VERY ANNOYING! There should be more information provided to consumers and other options for upgrades given. Appropriating my computer AND my broadband usage without adequate warning - there were no more reminders given, I just found I was being upgraded on the 17th of December - is akin to Windows barging into your house, tinkering with your computer, and pulling some cash (it will vary depending on what you are charged by your internet provider) out of your wallet!
I have now set my laptop to not automatically install updates - which I note Windows do not recommend.