Is it possible to prevent specific backups from getting deleted?
Based on how Time Machine consolidates it backups, unless there is an unpublished method to modify how Time Machine is coded to do this (and I don't think there is), I'd say no, you cannot prevent specific backups from getting deleted.

That said, why are you trying to preserve specific backups, to what end and do you only need files from your Home Folder or the entire Macintosh HD? 

Additionally, Time Machine backups are not bootable and if what you're looking for is a full bootable backup of the systems at a given point in time, you need to make a bootable clone of the drive at that time. You can use SuperDuper!, Carbon Copy Cloner or from the Recovery HD, use Disk Utility or in Terminal from the Recovery HD use hdiutil, etc.