as far, as I know, the trash is owned by the user. I guess, the only way, will be, to change the owner of each users trash and set up the preferences, that the user cannot put anything in the trash. BUT: It the user does that not with the GUI, he will still be able to delete files via the terminal. In my opinion, such a restriction does not make any sense

I'd 2nd what  said. Plus users always have the option of doing an Option-Command-Delete to immediately delete the current selection in the Finder (if it's files/folders they own) and locking the user's Trash folder won't stop that.

Also, if the users are admins, they can delete anything in the terminal, etc...

not helpful I know.. but I am just imagining what is going on with this user that your are trying to engineer such a solution. If they are a local admin, you have already given them the rights to do so, you could make a work flow to make an app/config/pref "self healing"  with smart groups. if they aren't and you are trying to protect stuff in the in the users space it sounds like a whole other problem.