TIME MACHINE backups

I'm hoping I can articulate my dilemma and question clearly and succinctly; it has been a while since the fiasco began. Fingers crossed. My external HD was nearly out of storage space, almost entirely due to Time Machine backups. I researched all the methods of erasing old backups no longer needed (the majority of them.) I began deleting old backups the standard way- one at a time- through Time Machine itself. Like many (judging from the many comment threads about this in forums and message boards) I found the deletion process inexplicably slow and laborious (in some cases never hearing the telltale haptic that indicates the task is complete...) So (like many) I resorted to simply accessing my external HD and manually trashing the old backups. This, of course, freed up the needed space on my HD. The problem NOW is that my Mac believes the items in the trash are 'in use' due to their file names ( system files, etc. ) Since I first trashed the backups, I've been unable to completely empty my trash; I've had to 'skip' the phantom system files or press 'continue' fifty times to trash the one or two items I've thrown away. Anyone know how to permanently delete the OLD system files my Mac believes are 'in use' but are really simply files from ancient backups?? Thanks!