I've pinned Virtualbox 5.1.22 in Synaptic after installing the .deb file manually. However as you can see the package remains marked as upgradable in the tray icon for system updates- wants to go to 5.1.24 as is available in the ppa.

I've temporarily disabled the virtualbox.org repo to remedy this for now. Would like a better solution.

Screenshots:
https://imgur.com/a/ITKjj