That won’t re-install an already installed package, you need the --reinstall option to force it to reinstall. Try:
sudo apt-get install rbp2-image-3.18.13-1-osmc --reinstall
By the way, the way kernel updates are installed on OSMC, you can receive and install other updates without the kernel being forced back to the “current” version, only if we push a still-newer kernel would your kernel then upgrade again, but you could then still revert it using the command above.
Another way to stop kernel updates being installed altogether would be the hold the rbp2-kernel-osmc meta-package like so:
sudo apt-mark hold rbp2-kernel-osmc
This would allow other parts of the system except the kernel to update. Then when you are ready to start receiving kernel updates again you can unhold the package:
sudo apt-mark unhold rbp2-kernel-osmc
In general we don’t recommend holding OSMC packages as there may be some unintended side effects of all of the OSMC packages in the system not updating as a whole, however if you have no other choice this might be a workable temporary solution for you.