There is a registry setting named AutoUpdateNotification, and if you change it, you should be able to disable the automatic update feature. It seems to be an undocumented, hidden feature, which may not work.
I'd advise you to download
Plus! Tweak! for Plus! 3.20 to change this registry setting. Download the program (it's an attachment in the first post), click the arrow below "Edit settings for this user" and select "--> All users <--". Click AutoUpdateNotification and change its value to False.
Supposedly this will work
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