boot - How to remove most recent kernel / how to stop old kernels being removed?

The 3 most recent kernels will not boot and I don't know why. My concern is that my only remaining old kernel will soon be automatically removed and then I have nothing. How can I remove the new kernels and more importantly, prevent the one older kernel from being removed automatically? Or, ideally, why are the new kernels failing? I have a blank screen. No error whatsoever. Where do I even start to investigate that?

Ubuntu 18.04.3 LTS Kernel (working) 4.15.0-20-generic x86_64 Asus E402M

Is it possible to prevent new kernels being installed? I keep the to LTS versions to avoid these failures!

lspci -knn | grep -EA3 'VGA|3D|Display'
00:02.0 VGA compatible controller [0300]: Intel Corporation Atom Processor Z36xxx/Z37xxx Series Graphics & Display [8086:0f31] (rev 0e)
    Subsystem: ASUSTeK Computer Inc. Atom Processor Z36xxx/Z37xxx Series Graphics & Display [1043:1aad]
    Kernel driver in use: i915
    Kernel modules: i915

00:14.0 USB controller [0c03]: Intel Corporation Atom Processor Z36xxx/Z37xxx, Celeron N2000 Series USB xHCI [8086:0f35] (rev 0e)

Instead of freezing an old kernel I would suggest to move to a NEW major kernel by running:

sudo apt-get install --install-recommends linux-generic-hwe-18.04 xserver-xorg-hwe-18.04

That will install kernel 5.0 to your system and new graphics stack.

You will still be able to boot with the old kernel using grub.

If this doesn't help, most likely the problem is not with kernels, but with something else in your system thet prevents new kernels from installing properly.