Multiple kernels, grub, free space /boot, CentOS

In this scenario you have to run yum remove and some base number of your kernel:
Now type:

yum remove kernel-2.6.32

This will output:

Loaded plugins: fastestmirror
Setting up Remove Process
Skipping the running kernel: kernel-2.6.32-573.12.1.el6.x86_64
Resolving Dependencies
--> Running transaction check
---> Package kernel.x86_64 0:2.6.32-504.el6 will be erased
---> Package kernel.x86_64 0:2.6.32-573.8.1.el6 will be erased
--> Finished Dependency Resolution

Dependencies Resolved

===================================================================================================================
 Package        Arch           Version                      Repository                                        Size
===================================================================================================================
Removing:
 kernel         x86_64         2.6.32-504.el6               @anaconda-CentOS-201410241409.x86_64/6.6         124 M
 kernel         x86_64         2.6.32-573.8.1.el6           @updates                                         126 M

Transaction Summary
===================================================================================================================
Remove        2 Package(s)

Installed size: 250 M

If you look very closely you can see that only two oldest kernels will be removed.