I've clicked don't upgrade, I've clicked any possible button, but I still get that annoying modal box asking me to upgrade. How do I stop it?
Just follow these steps to stop that annoying message.
Execute following command to open Software Sources in terminal
Go to Updates tab and uncheck all four checkboxes their. and select Never for
Automatically check for updates:option. Unchecking the checkboxes would be optional
Close the window.
Execute this command to update the repository:
sudo apt-get update
After that you'll not get any message. If you want to get again such message just follow the same procedure and do the opposite.
Hope this short description help you.. Reply if you need any further assistance..