Hello,
I'm affraid you cannot chest/delete files directly at a Time machine backup folder. Even if you have succeeded, I don't know how the Time machine would behave in a case you would need to do a restore.
As long as the viruses are only in a Time machine backup, they are harmless and I would personally leave them there.
To Taregreen: If you deleted the backup, rescanned the computer and it was clean and after a new backup you can detect viruses in a sparse bundle again, it looks as a false positive. Viruses are detected based on their signatures and cetain backups/virtual machine images can sometimes hit the signature as well (unfortunatelly, not deterministically).
Martin