In order to prevent misunderstandings, you should never try to remove a kernel while it is running.
You can always check which kernel is running by using the following command in the terminal:
The output may look as follows:
The following kernels are installed in your system:
In the above example, the system is running kernel version 4.14.74-1; 4.16 is available but not being used. In order to get rid of 4.16, the command should be
sudo mhwd-kernel -r linux416
See Removing Kernels in the Manjaro Wiki.
I am running kernel 4.14.74-1 on my own system, and even though the Manjaro Settings Manager tells me that newer kernel versions are available (e.g. 4.16.18_rt12-1, 4.17.19-1 and 4.18.12-1), these don't get installed unless I push the Install button.
I never changed
/etc/pacman.conf to achieve this, since the part quoted in GAD3R's answer reads (on my system):
# Pacman won't upgrade packages listed in IgnorePkg and members of IgnoreGroup