I'm trying to delete a container that I don't own. I log into designer as repos_manager which I thought had privileges to delete any object but it says I can't delete container because it is owned by someone else. How do I delete objects from designer if I don't own them?
The repos_manager id is usually not used for anything except managing the repository. You would only use that id when you are using the Repository Administration Utility (RAU). So you need to create another id that has the authority you need.
To do this, have a look and the Installation Guide under Creating Subordinate Users:
Grant that user the delete authority you want.