If you’re actually talking about IP, you don’t have to do anything.
Unless you sign a contract that transfers IP rights of things to another party, you keep rights to IP that you create without doing anything.
If you’re talking about protecting the files so that they can’t physically make copies that they then use in another project, there’s also nothing you can do. If you give them access so that they can fix issues with them, they’d be able to make copies of them.
Generally through contracts and lawyers. To actually do anything if you suspect/know that someone is doing something they shouldn’t with your content generally involves suing them. Or at least threatening to sue them. If you don’t have the resources to do that, it’s sort of a moot point.