First of all, I don’t recommend disabling Automatic Updates in general. It’s important to have system up to date, applications and kernel.
But there are several reasons why you might want to update your Kernel by hand, in time when you are ready to deal with problems. One most common reasons are performance graphics cards. It depends on manufacturer and model but sometimes it’s not very easy and stable to build graphics drivers, even when you are using DKMS.
If your computer is a tool for your job it’s very annoying to come to your PC in the morning and find it broken. You have to spent lot of time to fix it and it happends always when you don’t have time for this.
You probably already know, that there is unattended upgrade. Unattended upgrades check for new version of packages but, kernel packages don’t have never versions. Every kernel has package with version in name of the package like
ii linux-image-4.15.0-88-generic ii linux-image-4.15.0-91-generic
There is another 2 packages which describe actual kernel version in the system.
If you want to disable automatic upgrades of the kernel, just uninstall these two packages.
apt-get remove linux-image linux-image-generic
It’s not kernel itself, just packages which describe which kernel package should be installed on the system.
If you are in situation where you have to disable automatic kernel upgrade, upgrade it manualy once a week or month.
Simply install again
apt-get update && apt-get install -y linux-image-generic
After this operation, if everything works you now should again uninstall linux-image-generic to disable auto upgrade.