With Proxmox 5-1-46 I disabled update like this:
nano /etc/cron.d/pveupdate
# automatically generated - do not edit
#44 5 * * * root /usr/bin/pveupdate
With Proxmox 5-2-5 I do not find how to disable the script to avoid following error:
TASK ERROR: command 'apt-get update' failed: exit code 100