Hello,
do you have installed Admin Tools, too?
Just search for System plugins, you should find a couple of system plugin that are in charge of sending the updates.
Simply disable them and you'll be fine.
Just as a side info: we found out that in some environments Joomla fails to store the last run of the plugin, causing a huge load of mails sent out.
I'm currently working for a fix, so you'll be able to find it in the next version.
Davide Tampellini
Developer and Support Staff
🇮🇹Italian: native 🇬🇧English: good • 🕐 My time zone is Europe / Rome (UTC +1)
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!