Is it possible to prevent the root user from deleting the kernel?
I'm not sure why you'd want to. If it's to prevent accidents, you could protect any file using:
It IS going to cause issues when updating your kernel packages, though and frankly I don't see the point.
You can also mount your /boot partition as read-only and remount it as read-write only when upgrading kernel packages.
If it's because you don't trust people who have 'root' account access, then why would you grant a privilege that effectively allows the person to nuke your systems in 101 different ways to somebody you don't trust?
PS. Granted, there are other solutions that might allow you to define additional authorization methods but those are much more complicated and I don't think your case justifies their use.