Other than ridding yoru steamapps folder of the usernamed directories, you'll have to check each game. How its storing multiple user names for yourself and how it stores info for each steam user. Should be subfolders or files with their username floating about. Maybe before you start deleting stuff, make a backup of the whole steamapps folder just in case until you have things as you want them.

Deleting local content will delete it for everyone using that pc

In the library list section, right click on one of the games and select set category, then create a category - call it Z-OtherPeoplesGames or something like that - Z at the front to keep it at the bottom of the category list. Add all the games that belong to other people to that category.

Thanks for replies and for the workaround!