Hi @nephilim,
As your distro uses systemd there is no need to remove /etc/cron.d/certbot, it executes nothing if it detects that systemd is working on your system but you can remove it, no problem.
To stop/disable the certbot systemd timer.
systemctl stop certbot.timer
systemctl disable certbot.timer
And to be sure that in case a new debian certbot update doesn't activate certbot.timer again you could mask the certbot.timer.
systemctl mask certbot.timer
This mask creates a symlink from /etc/systemd/system/certbot.timer to /dev/null so this timer will run nothing.
Cheers,
sahsanu