Can't uninstall games because linked to steam?

[Rosadon]

This computer was originally my cousin's, he used steam, and some of his old steam games are still downloaded on the computer. Whenever i try to uninstall them under the Programs and features, it just opens up steam.
I cannot find anything in my steam that shows the games, despite them being on the computer but not in /my/ account.
I went through and deleted the games folders from the Steam/commomapp folder, but the it still exists within the programs.

[Bad 💀 Motha]

There is no need to uninstall Steam Games in that manner, just delete from the Steam folder.
Even a normal uninstall method for most Steam games would leave files behind. So you might still have a need to look in other places game files might be left behind and delete those as well, such as within My Documents or AppData structures.

If that is not your PC, then wipe the whole thing clean and start over with a clean OS.

You can then use app such as CCleaner to remove the entry still listed in Programs and Features; as well as clean up the Registry to remove any dead-links from now removed apps.