Help me Stop Mojave Auto Install Attempts

Hi there, I'm having a problem with my Mac Pro Mid 2010 -

I have it running on High Sierra, hardware limitations prevent me from upgrading to Mojave, which is fine for me, but my App store was set to auto-update stuff so it downloaded the install for Mojave, no problem here, but I decided to DELETE the Mojave Install file as it was taking up 5GB+ space and I don't have any use for it.

As soon as I deleted this file, my computer is booting to an automatic "InstallOS" drive that is flagging an error since the file is removed.

If I re-download the Mojave Install and just leave it in my Applications folder, my mac boots normally to High Sierra without any apparent install attempts.

I'd like to be able to remove this file and have my mac booting normally, does anyone know how I can stop my computer from triggering that install attempt at boot?

thanks,

j