On Linux that can be done, but not OS X, no program that I know of exists, but it would certainly be welcome, but Apple would likely sue them.
I did not realize while purchasing it, I will need installation DVD.
The original 10.6 machine specific disks should have came with the used machine, contact the original owner.
so be on safe side want to get rid of, if any remote access to my MacBook, data sharing or anything like this.
If I cannot make bootable USB, what settings/preferences I need to clean up for achieving my safety concern?
You will need to securely wipe the entire drive to accomplish this and you will need to boot from the 10.6 install disk.
Software can lie to you, so if it says one thing it still might be open and exposing you to intrusions.
I don't want to buy installation DVD from apple.
You'll need the 10.6.4-10.6.8 disk and if the machine originally came with 10.6.3 or later then you will need to contact Apple to get machine specific versions of the disks as those are not sold online, except if your lucky and find a used one for that exact model of Mac on eBay or something.
Intel Mac's that originally came with 10.4-10.6.2 from the factory can use the white 10.6.3 retail disks (sans free iLife), you can purchase from Apple or on Amazon.com or elsewhere.
If your interested in not installing malware, then don't use the illegal copies on P2P networks.
To find out what OS X version came with the machine, use the free MacTracker, also these links
To erase and install Snow Leopard
Optional: For Snow Leopard Speed Freaks