How do I uninstall Snow Leopard without install disk?

If you do not want to destroy the hard drive and as long as you don't have a MB 1,1, then you may be able to use a Parted Magic utility boot disk to zero out the contents of the hard drive. You can use Etcher (Mac/Windows/Linux) to create a bootable Parted Magic USB drive. Etcher requires macOS 10.9+. Option Boot the Parted Magic USB drive and select the orange icon labeled "EFI". There should be an icon on the Parted Magic desktop for erasing drives. You can get the last free version of Parted Magic from the Major Geeks site:

https://www.majorgeeks.com/mg/getmirror/parted_magic,1.html

Although I've not had to use it in several years, there was a way without installer media

to start up a mac in Target Disk so it would be seen by another mac, as though it were

an external hard drive. ~ In this state, you could use another mac w/ USB or FW ports.

• Use target disk mode to move files to another computer - Apple Support

https://support.apple.com/en-us/HT201462

This also had earlier history with FireWire instructions; slightly different though similar

where Target's disk could be erased; the target mac was started, and seen as hard drive.

Remove the hard drive from the device and destroy it.

you can always go to Disk utility in Applications and just Erase the entire disk. Should be adequate if you throw it away unless you have multi million dollar trade secrets, but then you would have IT and all that. You can unscrew ( with a tiny special screwdriver) the back underside panel and identify the harddrive, and smash it several times with a hammer. If you don't know what the hard drive looks like, google it or it looks like this https://www.macpartsdepot.com/661-3854-100gb-5400-sata-hard-drive-core-duo-15inch-macbook-pro.html?utm_campaign=1486655732&utm_source=google&utm_medium=cpc&utm_content=284484671429&utm_term=&adgroupid=56927109505&gclid=EAIaIQobChMIpKSysc6_5gIVTv_jBx3QKgfNEAQYASABEgLWRPD_BwE

I personally like to keep old harddrives around for future use. There are ways to erase which take several hours which make it impossible for even the best experts to read the data, but I doubt you need that. Just erase it with disk utility.