It's a fairly common scenario, you've just got your system set up the way you like it and then there's a new release. Your update manager starts nagging that you should upgrade, no matter how many times you dismiss it it just won't get the message!
You can disable this using the method below, but don't stick on old versions for too long!
nano /etc/update-manager/release-upgrades #change Prompt=normal to Prompt=never Ctrl-X, Y
Job done, just keep in mind that you'll actually have to remember to upgrade! Standard updates and bugfixes will still come down, but upgrades to the next version of the distribution won't.